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Add FAQ entry to mention using COALESCE() for concatenation of possible

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NULLs.
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Bruce Momjian 2006-12-11 22:44:53 +00:00
parent 9fa12ddda6
commit 051b52c886
2 changed files with 30 additions and 11 deletions
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18
doc/FAQ
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@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Tue Dec 5 18:13:32 EST 2006
Last updated: Mon Dec 11 17:44:33 EST 2006
Current maintainer: Bruce Momjian (bruce@momjian.us)
@ -59,8 +59,9 @@
4.8) How do I perform regular expression searches and case-insensitive
regular expression searches? How do I use an index for
case-insensitive searches?
4.9) In a query, how do I detect if a field is NULL? How can I sort on
whether a field is NULL or not?
4.9) In a query, how do I detect if a field is NULL? How do I
concatenate possible NULLs? How can I sort on whether a field is NULL
or not?
4.10) What is the difference between the various character types?
4.11.1) How do I create a serial/auto-incrementing field?
4.11.2) How do I get the value of a SERIAL insert?
@ -631,15 +632,22 @@
upper and lowercase characters, it can not have identical values that
differ only in case. To force a particular case to be stored in the
column, use a CHECK constraint or a trigger.
4.9) In a query, how do I detect if a field is NULL? How do I
concatenate possible NULLs? How can I sort on whether a field is NULL
or not?
4.9) In a query, how do I detect if a field is NULL? How can I sort on
whether a field is NULL or not?
4.9) In a query, how do I detect if a field is NULL? How do I concatenate
possible NULLs? How can I sort on whether a field is NULL or not?
You test the column with IS NULL and IS NOT NULL, like this:
SELECT *
FROM tab
WHERE col IS NULL;
To concatentate with possible NULLs, use COALESCE(), like this:
SELECT COALESCE(col1, '') || COALESCE(col2, '')
FROM tab
To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers
in your ORDER BY clause. Things that are true will sort higher than
things that are false, so the following will put NULL entries at the

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doc/src/FAQ/FAQ.html
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@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Tue Dec 5 18:13:32 EST 2006</P>
<P>Last updated: Mon Dec 11 17:44:33 EST 2006</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:bruce@momjian.us">bruce@momjian.us</A>)
@ -86,8 +86,8 @@
searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?<BR>
<A href="#item4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?<BR>
is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
How can I sort on whether a field is <SMALL> NULL</SMALL> or not?<BR>
<A href="#item4.10">4.10</A>) What is the difference between the
various character types?<BR>
<A href="#item4.11.1">4.11.1</A>) How do I create a
@ -823,10 +823,14 @@ table?</TD><TD>unlimited</TD></TR>
identical values that differ only in case. To force a particular
case to be stored in the column, use a <SMALL>CHECK</SMALL>
constraint or a trigger.</P>
<A href="#item4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
How can I sort on whether a field is <SMALL> NULL</SMALL> or not?<BR>
<H3 id="item4.9">4.9) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?</H3>
is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
How can I sort on whether a field is <SMALL> NULL</SMALL> or not?</H3>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>, like this:</P>
@ -837,6 +841,13 @@ table?</TD><TD>unlimited</TD></TR>
WHERE col IS NULL;
</PRE>
<P>To concatentate with possible <SMALL>NULL</SMALL>s, use <I>COALESCE()</I>,
like this:</P>
<PRE>
SELECT COALESCE(col1, '') || COALESCE(col2, '')
FROM tab
</PRE>
<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
Things that are <I>true</I> will sort higher than things that are <I>false</I>,