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doc: improve USING and NATURAL JOIN descriptions

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Patch by David G Johnston
This commit is contained in:
Bruce Momjian 2014-10-13 15:39:49 -04:00
parent 98aed6c721
commit 032f3b7e16
1 changed files with 43 additions and 14 deletions
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57
doc/src/sgml/queries.sgml
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@ -245,25 +245,43 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
condition: it takes a Boolean value expression of the same
kind as is used in a <literal>WHERE</> clause. A pair of rows
from <replaceable>T1</> and <replaceable>T2</> match if the
<literal>ON</> expression evaluates to true for them.
<literal>ON</> expression evaluates to true.
</para>
<para>
<literal>USING</> is a shorthand notation: it takes a
comma-separated list of column names, which the joined tables
must have in common, and forms a join condition specifying
equality of each of these pairs of columns. Furthermore, the
output of <literal>JOIN USING</> has one column for each of
the equated pairs of input columns, followed by the
remaining columns from each table. Thus, <literal>USING (a, b,
c)</literal> is equivalent to <literal>ON (t1.a = t2.a AND
t1.b = t2.b AND t1.c = t2.c)</literal> with the exception that
if <literal>ON</> is used there will be two columns
<literal>a</>, <literal>b</>, and <literal>c</> in the result,
whereas with <literal>USING</> there will be only one of each
(and they will appear first if <command>SELECT *</> is used).
The <literal>USING</> clause allows you to take advantage of
the specific situation where both sides of the join use the
same name for the joining columns. It takes a
comma-separated list of the shared column names
and forms a join using the equals operator. Furthermore, the
output of <literal>JOIN USING</> has one column for each of the
listed columns, followed by the remaining columns from each table.
</para>
<para>The output column difference between <literal>ON</> and
<literal>USING</> when invoking <literal>SELECT *</> is:</para>
<itemizedlist>
<listitem>
<para>
<literal>ON</> - all columns from <replaceable>T1</> followed
by all columns from <replaceable>T2</>
</para>
</listitem>
<listitem>
<para>
<literal>USING</> - all join columns, one copy each
and in the listed order, followed by non-join columns
in <replaceable>T1</> followed by non-join columns in
<replaceable>T2</>
</para>
</listitem>
<listitem>
<para>
Examples provided below
</para>
</listitem>
</itemizedlist>
<para>
<indexterm>
<primary>join</primary>
@ -281,6 +299,17 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
<literal>CROSS JOIN</literal>.
</para>
<note>
<para>
<literal>USING</literal> is reasonably safe from column changes
in the joined relations since only the specific columns mentioned
are considered. <literal>NATURAL</> is considerably more problematic
if you are referring to relations only by name (views and tables)
since any schema changes to either relation that cause a new matching
column name to be present will cause the join to consider that new column.
</para>
</note>
<para>
The possible types of qualified join are: