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Documentation for VALUES lists. Joe Conway and Tom Lane

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Tom Lane
2006-09-18 19:54:01 +00:00
parent 5f04ce3126
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doc/src/sgml/queries.sgml
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@@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.35 2006/02/18 23:14:45 neilc Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.36 2006/09/18 19:54:01 tgl Exp $ -->
<chapter id="queries">
<title>Queries</title>
@@ -104,7 +104,7 @@ SELECT random();
produce a virtual table that provides the rows that are passed to
the select list to compute the output rows of the query.
</para>
<sect2 id="queries-from">
<title>The <literal>FROM</literal> Clause</title>
@@ -253,12 +253,12 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
<para>
<indexterm>
<primary>join</primary>
<secondary>natural</secondary>
</indexterm>
<primary>join</primary>
<secondary>natural</secondary>
</indexterm>
<indexterm>
<primary>natural join</primary>
</indexterm>
<primary>natural join</primary>
</indexterm>
Finally, <literal>NATURAL</> is a shorthand form of
<literal>USING</>: it forms a <literal>USING</> list
consisting of exactly those column names that appear in both
@@ -511,33 +511,36 @@ SELECT * FROM some_very_long_table_name s JOIN another_fairly_long_name a ON s.i
<programlisting>
SELECT * FROM my_table AS m WHERE my_table.a &gt; 5;
</programlisting>
is not valid SQL syntax. What will actually happen (this is a
<productname>PostgreSQL</productname> extension to the standard)
is that an implicit table reference is added to the
is not valid according to the SQL standard. In
<productname>PostgreSQL</productname> this will draw an error if the
<xref linkend="guc-add-missing-from"> configuration variable is
<literal>off</>. If it is <literal>on</>, an implicit table reference
will be added to the
<literal>FROM</literal> clause, so the query is processed as if
it were written as
<programlisting>
SELECT * FROM my_table AS m, my_table AS my_table WHERE my_table.a &gt; 5;
</programlisting>
which will result in a cross join, which is usually not what you
want.
That will result in a cross join, which is usually not what you want.
</para>
<para>
Table aliases are mainly for notational convenience, but it is
necessary to use them when joining a table to itself, e.g.,
<programlisting>
SELECT * FROM my_table AS a CROSS JOIN my_table AS b ...
SELECT * FROM people AS mother JOIN people AS child ON mother.id = child.mother_id;
</programlisting>
Additionally, an alias is required if the table reference is a
subquery (see <xref linkend="queries-subqueries">).
</para>
<para>
Parentheses are used to resolve ambiguities. The following
statement will assign the alias <literal>b</literal> to the
result of the join, unlike the previous example:
Parentheses are used to resolve ambiguities. In the following example,
the first statement assigns the alias <literal>b</literal> to the second
instance of <literal>my_table</>, but the second statement assigns the
alias to the result of the join:
<programlisting>
SELECT * FROM my_table AS a CROSS JOIN my_table AS b ...
SELECT * FROM (my_table AS a CROSS JOIN my_table) AS b ...
</programlisting>
</para>
@@ -592,6 +595,17 @@ FROM (SELECT * FROM table1) AS alias_name
reduced to a plain join, arise when the subquery involves
grouping or aggregation.
</para>
<para>
A subquery can also be a <command>VALUES</> list:
<programlisting>
FROM (VALUES ('anne', 'smith'), ('bob', 'jones'), ('joe', 'blow'))
AS names(first, last)
</programlisting>
Again, a table alias is required. Assigning alias names to the columns
of the <command>VALUES</> list is optional, but is good practice.
For more information see <xref linkend="queries-values">.
</para>
</sect3>
<sect3 id="queries-tablefunctions">
@@ -814,7 +828,7 @@ SELECT <replaceable>select_list</replaceable>
(3 rows)
</screen>
</para>
<para>
In the second query, we could not have written <literal>SELECT *
FROM test1 GROUP BY x</literal>, because there is no single value
@@ -1194,7 +1208,7 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
<indexterm zone="queries-order">
<primary>ORDER BY</primary>
</indexterm>
<para>
After a query has produced an output table (after the select list
has been processed) it can optionally be sorted. If sorting is not
@@ -1335,4 +1349,74 @@ SELECT <replaceable>select_list</replaceable>
</para>
</sect1>
<sect1 id="queries-values">
<title><literal>VALUES</literal> Lists</title>
<indexterm zone="queries-values">
<primary>VALUES</primary>
</indexterm>
<para>
<literal>VALUES</> provides a way to generate a <quote>constant table</>
that can be used in a query without having to actually create and populate
a table on-disk. The syntax is
<synopsis>
VALUES ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) [, ...]
</synopsis>
Each parenthesized list of expressions generates a row in the table.
The lists must all have the same number of elements (i.e., the number
of columns in the table), and corresponding entries in each list must
have compatible datatypes. The actual datatype assigned to each column
of the result is determined using the same rules as for <literal>UNION</>
(see <xref linkend="typeconv-union-case">).
</para>
<para>
As an example,
<programlisting>
VALUES (1, 'one'), (2, 'two'), (3, 'three');
</programlisting>
will return a table of two columns and three rows. It's effectively
equivalent to
<programlisting>
SELECT 1 AS column1, 'one' AS column2
UNION ALL
SELECT 2, 'two'
UNION ALL
SELECT 3, 'three';
</programlisting>
By default, <productname>PostgreSQL</productname> assigns the names
<literal>column1</>, <literal>column2</>, etc. to the columns of a
<literal>VALUES</> table. The column names are not specified by the
SQL standard and different database systems do it differently, so
it's usually better to override the default names with a table alias
list.
</para>
<para>
Syntactically, <literal>VALUES</> followed by expression lists is
treated as equivalent to
<synopsis>
SELECT <replaceable>select_list</replaceable> FROM <replaceable>table_expression</replaceable>
</synopsis>
and can appear anywhere a <literal>SELECT</> can. For example, you can
use it as an arm of a <literal>UNION</>, or attach a
<replaceable>sort_specification</replaceable> (<literal>ORDER BY</>,
<literal>LIMIT</>, and/or <literal>OFFSET</>) to it. <literal>VALUES</>
is most commonly used as the data source in an <command>INSERT</> command,
and next most commonly as a subquery.
</para>
<para>
For more information see <xref linkend="sql-values"
endterm="sql-values-title">.
</para>
</sect1>
</chapter>