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Enhance documentation of date/time functions. Add extract, current_date,
current_time, current_timestamp. Add more examples. Remove age(), because it doesn't work like it is documented, and no one could explain it to me.
This commit is contained in:
Peter Eisentraut
parent
b9c7d371ed
commit
48d03278a2
@ -1,4 +1,4 @@
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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.51 2001/02/12 14:21:27 momjian Exp $ -->
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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.52 2001/02/18 17:17:17 petere Exp $ -->
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<chapter id="functions">
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<title>Functions and Operators</title>
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@ -17,8 +17,8 @@
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If you are concerned about portability then take note that most of
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the functions and operators described in this chapter, with the
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exception of the most trivial arithmetic and comparison operators
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and some explicitly marked functions, are not specified by the SQL
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standard. However, many other RDBMS packages provide a lot of the
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and some explicitly marked functions, are not specified by the <acronym>SQL</acronym>
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standard. However, many other <acronym>RDBMS</acronym> packages provide a lot of the
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same or similar functions, and some of the ones provided in
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<productname>Postgres</productname> have in fact been inspired by
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other implementations.
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@ -37,7 +37,7 @@
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<member>NOT</member>
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</simplelist>
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SQL uses a three-valued boolean logic where NULL represents
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<acronym>SQL</acronym> uses a three-valued Boolean logic where NULL represents
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<quote>unknown</quote>. Observe the following truth tables:
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<informaltable>
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@ -179,9 +179,9 @@
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<para>
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Comparison operators are available for all data types where this
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makes sense. All comparison operators are binary operators that
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return boolean values; expressions like <literal>1 < 2 <
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3</literal> are not valid (because there is no
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<literal><</literal> operator to compare a boolean with
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return values of type <type>boolean</type>; expressions like
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<literal>1 < 2 < 3</literal> are not valid (because there is
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no <literal><</literal> operator to compare a Boolean value with
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<literal>3</literal>).
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</para>
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@ -685,7 +685,7 @@
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</para>
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<para>
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SQL defines some string functions with a special syntax where
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<acronym>SQL</acronym> defines some string functions with a special syntax where
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certain keywords rather than commas are used to separate the
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arguments. Details are in <xref linkend="functions-string-sql">.
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These functions are also implemented using the regular syntax for
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@ -784,7 +784,7 @@
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<para>
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Additional string manipulation functions are available and are
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listed below. Some of them are used internally to implement the
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SQL string functions listed above.
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<acronym>SQL</acronym> string functions listed above.
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</para>
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<table id="functions-string-other">
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@ -1049,7 +1049,7 @@
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<para>
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The keyword <token>ILIKE</token> can be used instead of
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<token>LIKE</token> to make the match case insensitive according
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to the active locale. This is not in the SQL standard but is a
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to the active locale. This is not in the <acronym>SQL</acronym> standard but is a
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<productname>Postgres</productname> extension.
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</para>
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@ -1066,7 +1066,7 @@
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<sect2 id="functions-regexp">
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<title>POSIX Regular Expressions</title>
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<title><acronym>POSIX</acronym> Regular Expressions</title>
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<table>
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<title>Regular Expression Match Operators</title>
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@ -1106,7 +1106,7 @@
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</table>
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<para>
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POSIX regular expressions provide a more powerful means for
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<acronym>POSIX</acronym> regular expressions provide a more powerful means for
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pattern matching than the <function>LIKE</function> function.
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Many Unix tools such as <command>egrep</command>,
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<command>sed</command>, or <command>awk</command> use a pattern
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@ -1131,7 +1131,7 @@
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<!-- derived from the re_format.7 man page -->
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<para>
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Regular expressions (<quote>RE</quote>s), as defined in POSIX
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Regular expressions (<quote>RE</quote>s), as defined in <acronym>POSIX</acronym>
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1003.2, come in two forms: modern REs (roughly those of
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<command>egrep</command>; 1003.2 calls these
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<quote>extended</quote> REs) and obsolete REs (roughly those of
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@ -1379,7 +1379,7 @@
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The <productname>Postgres</productname> formatting functions
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provide a powerful set of tools for converting various data types
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(date/time, integer, floating point, numeric) to formatted strings
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and for converting from formatted strings to specific datetypes.
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and for converting from formatted strings to specific data types.
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These functions all follow a common calling convention: The first
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argument is the value to be formatted and the second argument is a
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template that defines the output format.
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@ -1713,7 +1713,7 @@
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<para>
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A double quote (<quote><literal>"</literal></quote>) between
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quotation marks is skipped and is not parsed. If you want to
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have a double quote in the output you must preceed it with a
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have a double quote in the output you must precede it with a
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double backslash, for example <literal>'\\"YYYY
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Month\\"'</literal>. <!-- " font-lock sanity :-) -->
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</para>
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@ -1829,7 +1829,7 @@
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but to_char(-12, 'MI9999') produces <literal>'- 12'</literal>.
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The Oracle implementation does not allow the use of
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<literal>MI</literal> ahead of <literal>9</literal>, but rather
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requires that <literal>9</literal> preceed
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requires that <literal>9</literal> precede
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<literal>MI</literal>.
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</para>
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</listitem>
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@ -2027,124 +2027,647 @@
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<title>Date/Time Functions</title>
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<para>
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The date/time functions provide a powerful set of tools
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for manipulating various date/time types.
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<xref linkend="functions-datetime-table"> shows the available
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functions for date/time value processing. The basic arithmetic
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operators (<literal>+</literal>, <literal>*</literal>, etc.) are
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also available. For formatting functions, refer to <xref
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linkend="functions-formatting">. You should be familiar with the
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background information on date/time data types (see <xref
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linkend="datatype-datetime">).
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</para>
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<para>
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<table tocentry="1">
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<table id="functions-datetime-table">
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<title>Date/Time Functions</title>
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<tgroup cols="4">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Returns</entry>
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<entry>Name</entry>
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<entry>Return Type</entry>
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<entry>Description</entry>
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<entry>Example</entry>
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<entry>Result</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>abstime(timestamp)</entry>
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<entry>abstime</entry>
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<entry>convert to abstime</entry>
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<entry>abstime(timestamp 'now')</entry>
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</row>
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<![IGNORE[
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<!--
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These two functions don't seem to do what it says here, or anything
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reasonable at all for that matter.
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-->
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<row>
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<entry>age(timestamp)</entry>
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<entry>interval</entry>
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<entry>preserve months and years</entry>
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<entry>Subtract argument from current date, preserve years and months and days</entry>
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<entry>age(timestamp '1957-06-13')</entry>
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<entry>43 years 8 mons 3 days</entry>
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</row>
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<row>
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<entry>age(timestamp,timestamp)</entry>
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<entry>age(timestamp, timestamp)</entry>
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<entry>interval</entry>
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<entry>preserve months and years</entry>
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<entry>age('now', timestamp '1957-06-13')</entry>
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<entry></entry>
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</row>
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]]>
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<row>
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<entry>date_part(text,timestamp)</entry>
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<entry>double precision</entry>
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<entry>portion of date</entry>
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<entry>date_part('dow',timestamp 'now')</entry>
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<entry>current_date</entry>
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<entry>date</entry>
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<entry>
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returns current date; see also <link
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linkend="functions-datetime-current">below</link>
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</entry>
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<entry></entry>
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<entry></entry>
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</row>
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<row>
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<entry>date_part(text,interval)</entry>
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<entry>double precision</entry>
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<entry>portion of time</entry>
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<entry>date_part('hour',interval '4 hrs 3 mins')</entry>
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<entry>current_time</entry>
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<entry>time</entry>
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<entry>
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returns current time (of day); see also <link
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linkend="functions-datetime-current">below</link>
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</entry>
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<entry></entry>
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<entry></entry>
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</row>
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<row>
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<entry>date_trunc(text,timestamp)</entry>
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<entry>current_timestamp</entry>
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<entry>timestamp</entry>
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<entry>truncate date</entry>
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<entry>date_trunc('month',abstime 'now')</entry>
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<entry>
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returns current date and time; see also <link
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linkend="functions-datetime-current">below</link>
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</entry>
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<entry></entry>
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<entry></entry>
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</row>
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<row>
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<entry>interval(reltime)</entry>
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<entry>interval</entry>
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<entry>convert to interval</entry>
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<entry>interval(reltime '4 hours')</entry>
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<entry>date_part(text, timestamp)</entry>
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<entry>double precision</entry>
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<entry>
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extract subfield from date/time value (equivalent to
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<function>extract</function>); see also <link
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linkend="functions-datetime-datepart">below</link>
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</entry>
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<entry>date_part('hour', timestamp '2001-02-16 20:38:40')</entry>
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<entry>20</entry>
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</row>
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<row>
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<entry>date_part(text, interval)</entry>
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<entry>double precision</entry>
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<entry>
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extract subfield from interval value (equivalent to
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<function>extract</function>); see also <link
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linkend="functions-datetime-datepart">below</link>
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</entry>
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<entry>date_part('month', interval '2 years 3 months')</entry>
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<entry>3</entry>
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</row>
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<row>
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<entry>date_trunc(text, timestamp)</entry>
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<entry>timestamp</entry>
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<entry>
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truncate date to specified precision; see also <link
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linkend="functions-datetime-trunc">below</link>
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</entry>
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<entry>date_trunc('hour', timestamp '2001-02-16 20:38:40')</entry>
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<entry>2001-02-16 20:00:00+00</entry>
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</row>
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<row>
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<entry>extract(identifier from timestamp)</entry>
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<entry>double precision</entry>
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<entry>
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extract subfield from date/time value; see also <link
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linkend="functions-datetime-extract">below</link>
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</entry>
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<entry>extract(hour from timestamp '2001-02-16 20:38:40')</entry>
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<entry>20</entry>
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</row>
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<row>
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<entry>extract(identifier from interval)</entry>
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<entry>double precision</entry>
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<entry>
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extract subfield from interval value; see also <link
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linkend="functions-datetime-extract">below</link>
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</entry>
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<entry>extract(month from interval '2 years 3 months')</entry>
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<entry>3</entry>
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</row>
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<row>
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<entry>isfinite(timestamp)</entry>
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<entry>bool</entry>
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<entry>a finite time?</entry>
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<entry>isfinite(timestamp 'now')</entry>
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<entry>boolean</entry>
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<entry>Returns true if the time stamp is finite (neither invalid nor infinity)</entry>
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<entry>isfinite(timestamp '2001-02-16 21:28:30')</entry>
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<entry>true</entry>
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</row>
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<row>
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<entry>isfinite(interval)</entry>
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<entry>bool</entry>
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<entry>a finite time?</entry>
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<entry>isfinite(interval '4 hrs')</entry>
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<entry>boolean</entry>
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<entry>Returns true if the interval is finite in length</entry>
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<entry>isfinite(interval '4 hours')</entry>
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<entry>true</entry>
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</row>
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<row>
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<entry>reltime(interval)</entry>
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<entry>reltime</entry>
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<entry>convert to reltime</entry>
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<entry>reltime(interval '4 hrs')</entry>
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<entry>now()</entry>
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<entry>timestamp</entry>
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<entry>
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returns current date and time (equivalent to
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<function>current_timestamp</function>); see also <link
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linkend="functions-datetime-current">below</link>
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</entry>
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<entry></entry>
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<entry></entry>
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</row>
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<row>
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<entry>timestamp(date)</entry>
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<entry>timestamp</entry>
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<entry>convert to timestamp</entry>
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<entry>timestamp(date 'today')</entry>
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<entry>convert date to timestamp</entry>
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<entry>timestamp(date '2000-12-25')</entry>
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<entry>2000-12-25 00:00:00</entry>
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</row>
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<row>
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<entry>timestamp(date,time)</entry>
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<entry>timestamp(date, time)</entry>
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<entry>timestamp</entry>
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<entry>convert to timestamp</entry>
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<entry>timestamp(timestamp '1998-02-24',time '23:07');</entry>
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</row>
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<row>
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<entry>to_char(timestamp,text)</entry>
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<entry>text</entry>
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<entry>convert to string</entry>
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<entry>to_char(timestamp '1998-02-24','DD');</entry>
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<entry>combine date and time into a timestamp</entry>
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<entry>timestamp(date '1998-02-24',time '23:07')</entry>
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<entry>1998-02-24 23:07:00</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<sect2 id="functions-datetime-extract">
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<title><function>EXTRACT</function>, <function>date_part</function></title>
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<synopsis>
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EXTRACT (<replaceable>field</replaceable> FROM <replaceable>source</replaceable>)
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</synopsis>
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<para>
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The <function>extract</function> function retrieves sub-fields
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from date/time values, such as year or hour.
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<replaceable>source</replaceable> is a value expression that
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evaluates to type <type>timestamp</type> or <type>interval</type>.
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(Expressions of type <type>date</type> or <type>time</type> will
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be cast to <type>timestamp</type> and can therefore be used as
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well.) <replaceable>field</replaceable> is an identifier (not a
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string!) that selects what field to extract from the source value.
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The <function>extract</function> function returns values of type
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<type>double precision</type>.
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The following are valid values:
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<!-- alphabetical -->
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<variablelist>
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<varlistentry>
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<term>century</term>
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<listitem>
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<para>
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The year field divided by 100
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</para>
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<informalexample>
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<screen>
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SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
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<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
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</screen>
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</informalexample>
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<para>
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Note that this is not really the century that the date is in.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>day</term>
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<listitem>
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<para>
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The day (of the month) field (1 - 31)
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</para>
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<informalexample>
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<screen>
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SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
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<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
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</screen>
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</informalexample>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>decade</term>
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<listitem>
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<para>
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The year field divided by 10
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</para>
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<informalexample>
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<screen>
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SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
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<lineannotation>Result: </lineannotation><computeroutput>200</computeroutput>
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</screen>
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</informalexample>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>dow</term>
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<listitem>
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<para>
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The day of the week (0 - 6; Sunday is 0) (for
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<type>timestamp</type> values only)
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</para>
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<informalexample>
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<screen>
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SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
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<lineannotation>Result: </lineannotation><computeroutput>5</computeroutput>
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</screen>
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</informalexample>
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</listitem>
|
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</varlistentry>
|
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<varlistentry>
|
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<term>doy</term>
|
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<listitem>
|
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<para>
|
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The day of the year (1 - 365/366) (for <type>timestamp</type> values only)
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</para>
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<informalexample>
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<screen>
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SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
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<lineannotation>Result: </lineannotation><computeroutput>47</computeroutput>
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</screen>
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</informalexample>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>epoch</term>
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<listitem>
|
||||
<para>
|
||||
For <type>date</type> and <type>timestamp</type> values, the
|
||||
number of seconds since 1970-01-01 00:00:00 (Result may be
|
||||
negative.); for <type>interval</type> values, the total number
|
||||
of seconds in the interval
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>982352320</computeroutput>
|
||||
|
||||
SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>442800</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>hour</term>
|
||||
<listitem>
|
||||
<para>
|
||||
The hour field (0 - 23)
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>microseconds</term>
|
||||
<listitem>
|
||||
<para>
|
||||
The seconds field, including fractional parts, multiplied by 1
|
||||
000 000. Note that this includes full seconds.
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>28500000</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>millennium</term>
|
||||
<listitem>
|
||||
<para>
|
||||
The year field divided by 1000
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
|
||||
<para>
|
||||
Note that this is not really the millennium that the date is in.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>milliseconds</term>
|
||||
<listitem>
|
||||
<para>
|
||||
The seconds field, including fractional parts, multiplied by
|
||||
1000. Note that this includes full seconds.
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>28500</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>minute</term>
|
||||
<listitem>
|
||||
<para>
|
||||
The minutes field (0 - 59)
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>38</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>month</term>
|
||||
<listitem>
|
||||
<para>
|
||||
For <type>timestamp</type> values, the number of the month
|
||||
within the year (1 - 12) ; for <type>interval</type> values
|
||||
the number of months, modulo 12 (0 - 11)
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>
|
||||
|
||||
SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>
|
||||
|
||||
SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>quarter</term>
|
||||
<listitem>
|
||||
<para>
|
||||
The quarter of the year (1 - 4) that the day is in (for
|
||||
<type>timestamp</type> values only)
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>second</term>
|
||||
<listitem>
|
||||
<para>
|
||||
The seconds field, including fractional parts (0 -
|
||||
59<footnote><simpara>60 if leap seconds are
|
||||
implemented by the operating system</simpara></footnote>)
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>40</computeroutput>
|
||||
|
||||
SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>28.5</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>week</term>
|
||||
<listitem>
|
||||
<para>
|
||||
From a <type>timestamp</type> value, calculate the number of
|
||||
the week of the year that the day is in. By definition
|
||||
(<acronym>ISO</acronym> 8601), the first week of a year
|
||||
contains January 4 of that year. (The <acronym>ISO</acronym>
|
||||
week starts on Monday.) In other words, the first Thursday of
|
||||
a year is in week 1 of that year.
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>7</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term>year</term>
|
||||
<listitem>
|
||||
<para>
|
||||
The year field
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>2001</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<!--
|
||||
tz
|
||||
tz_hour
|
||||
tz_minute
|
||||
-->
|
||||
|
||||
</variablelist>
|
||||
|
||||
</para>
|
||||
|
||||
<para>
|
||||
For the
|
||||
<function>date_part</function> and <function>date_trunc</function>
|
||||
functions, arguments can be
|
||||
`<literal>year</literal>', `<literal>month</literal>',
|
||||
`<literal>day</literal>', `<literal>hour</literal>',
|
||||
`<literal>minute</literal>', and `<literal>second</literal>',
|
||||
as well as the more specialized quantities
|
||||
`<literal>decade</literal>', `<literal>century</literal>',
|
||||
`<literal>millennium</literal>', `<literal>millisecond</literal>',
|
||||
and `<literal>microsecond</literal>'.
|
||||
<function>date_part</function> allows `<literal>dow</literal>'
|
||||
to return day of week, '<literal>week</literal>' to return the
|
||||
ISO-defined week of year, and `<literal>epoch</literal>' to return
|
||||
seconds since 1970 (for <type>timestamp</type>)
|
||||
or '<literal>epoch</literal>' to return total elapsed seconds
|
||||
(for <type>interval</type>).
|
||||
The <function>extract</function> function is primarily intended
|
||||
for computational processing. For formatting date/time values for
|
||||
display, see <xref linkend="functions-formatting">.
|
||||
</para>
|
||||
|
||||
<anchor id="functions-datetime-datepart">
|
||||
<para>
|
||||
The <function>date_part</function> function is the traditional
|
||||
<productname>Postgres</productname> equivalent to the
|
||||
<acronym>SQL</acronym>-function <function>extract</function>:
|
||||
<synopsis>
|
||||
date_part('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
|
||||
</synopsis>
|
||||
Note that here the <replaceable>field</replaceable> value needs to
|
||||
be a string. The valid field values for
|
||||
<function>date_part</function> are the same as for
|
||||
<function>extract</function>.
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
|
||||
|
||||
SELECT date_part('hour', INTERVAL '4 hours 3 minutes')
|
||||
<lineannotation>Result: </lineannotation><computeroutput>4</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
|
||||
</sect2>
|
||||
|
||||
<sect2 id="functions-datetime-trunc">
|
||||
<title><function>date_trunc</function></title>
|
||||
|
||||
<para>
|
||||
The function <function>date_trunc</function> is conceptually
|
||||
similar to the <function>trunc</function> function for numbers.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
<synopsis>
|
||||
date_trunc('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
|
||||
</synopsis>
|
||||
<replaceable>source</replaceable> is a value expression of type
|
||||
<type>timestamp</type> (values of type <type>date</type> and
|
||||
<type>time</type> are cast automatically).
|
||||
<replaceable>field</replaceable> selects to which precision to
|
||||
truncate the time stamp value. The return value is of type
|
||||
<type>timestamp</type> with all fields that are less than the
|
||||
selected one set to zero (or one, for day and month).
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Valid values for <replaceable>field</replaceable> are:
|
||||
<simplelist>
|
||||
<member>microseconds</member>
|
||||
<member>milliseconds</member>
|
||||
<member>second</member>
|
||||
<member>minute</member>
|
||||
<member>hour</member>
|
||||
<member>day</member>
|
||||
<member>month</member>
|
||||
<member>year</member>
|
||||
<member>decade</member>
|
||||
<member>century</member>
|
||||
<member>millennium</member>
|
||||
</simplelist>
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<para>
|
||||
<screen>
|
||||
SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 20:00:00+00</computeroutput>
|
||||
|
||||
SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
|
||||
<lineannotation>Result: </lineannotation><computeroutput>2001-01-01 00:00:00+00</computeroutput>
|
||||
</screen>
|
||||
</para>
|
||||
</informalexample>
|
||||
</sect2>
|
||||
|
||||
<sect2 id="functions-datetime-current">
|
||||
<title>Current Date/Time</title>
|
||||
|
||||
<para>
|
||||
The following functions are available to select the current date and/or time:
|
||||
<synopsis>
|
||||
CURRENT_TIME
|
||||
CURRENT_DATE
|
||||
CURRENT_TIMESTAMP
|
||||
</synopsis>
|
||||
Note that because of the requirements of the
|
||||
<acronym>SQL</acronym> standard, these functions must not be
|
||||
called with trailing parentheses.
|
||||
</para>
|
||||
|
||||
<informalexample>
|
||||
<screen>
|
||||
SELECT CURRENT_TIME;
|
||||
<computeroutput>19:07:13</computeroutput>
|
||||
|
||||
SELECT CURRENT_DATE;
|
||||
<computeroutput>2001-02-17</computeroutput>
|
||||
|
||||
SELECT CURRENT_TIMESTAMP;
|
||||
<computeroutput>2001-02-17 19:07:32+00</computeroutput>
|
||||
</screen>
|
||||
</informalexample>
|
||||
|
||||
<para>
|
||||
The function <function>now()</function> is the traditional
|
||||
<productname>Postgres</productname> equivalent to
|
||||
<function>CURRENT_TIMESTAMP</function>.
|
||||
<productname>Postgres</productname> furthermore has special
|
||||
date/time <quote>constants</quote> that can be used to specify the
|
||||
current time. The following three all return the same result:
|
||||
<programlisting>
|
||||
SELECT CURRENT_TIMESTAMP;
|
||||
SELECT now();
|
||||
SELECT TIMESTAMP 'now';
|
||||
</programlisting>
|
||||
<note>
|
||||
<para>
|
||||
You do not want to use the third form when specifying a DEFAULT
|
||||
value when creating a table. The system will immediately
|
||||
evaluate the constant, thus when the default value is needed,
|
||||
the time of the table creation would be used! The first two
|
||||
forms will not be evaluated until the default value is used,
|
||||
because they are function calls.
|
||||
</para>
|
||||
</note>
|
||||
</para>
|
||||
</sect2>
|
||||
</sect1>
|
||||
|
||||
|
||||
@ -2335,13 +2858,13 @@
|
||||
</row>
|
||||
<row>
|
||||
<entry>isclosed(path)</entry>
|
||||
<entry>bool</entry>
|
||||
<entry>boolean</entry>
|
||||
<entry>a closed path?</entry>
|
||||
<entry>isclosed(path '((0,0),(1,1),(2,0))')</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry>isopen(path)</entry>
|
||||
<entry>bool</entry>
|
||||
<entry>boolean</entry>
|
||||
<entry>an open path?</entry>
|
||||
<entry>isopen(path '[(0,0),(1,1),(2,0)]')</entry>
|
||||
</row>
|
||||
@ -2704,7 +3227,7 @@ Not defined by this name. Implements the intersection operator '#'
|
||||
<title>Conditional Expressions</title>
|
||||
|
||||
<para>
|
||||
This section descibes the SQL-compliant conditional expressions
|
||||
This section describes the <acronym>SQL</acronym>-compliant conditional expressions
|
||||
available in <productname>Postgres</productname>.
|
||||
</para>
|
||||
|
||||
@ -2728,9 +3251,9 @@ END
|
||||
<para>
|
||||
The <acronym>SQL</acronym> <token>CASE</token> expression is a
|
||||
generic conditional expression, similar to if/else statements in
|
||||
other languages. <token>CASE</token> clauses can be used whereever
|
||||
other languages. <token>CASE</token> clauses can be used wherever
|
||||
an expression is valid. <replaceable>condition</replaceable> is an
|
||||
expression that returns a boolean result. If the result is true
|
||||
expression that returns a <type>boolean</type> result. If the result is true
|
||||
then the value of the <token>CASE</token> expression is
|
||||
<replaceable>result</replaceable>. If the result is false any
|
||||
subsequent <token>WHEN</token> clauses are searched in the same
|
||||
|
||||
Loading…
Reference in New Issue
Block a user