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<chapter id="sql-syntax">
<para>
SQL input consists of a sequence of
<firstterm>commands</firstterm>. A command is composed of a
- sequence of <firstterm>tokens</firstterm>, which depend on the
- syntax of the particular command, terminated by a semicolon
- (<quote>;</quote>). The end of the input stream also terminates a
- command.
+ sequence of <firstterm>tokens</firstterm>, terminated by a
+ semicolon (<quote>;</quote>). The end of the input stream also
+ terminates a command. Which tokens are valid depends on the syntax
+ of the particular command.
</para>
<para>
<informalexample id="sql-syntax-ex-commands">
<para>
- For example, the following is (lexically) valid SQL input:
+ For example, the following is (syntactically) valid SQL input:
<programlisting>
SELECT * FROM MY_TABLE;
UPDATE MY_TABLE SET A = 5;
key word can be letters, digits
(<literal>0</literal>-<literal>9</literal>), or underscores,
although the SQL standard will not define a key word that contains
- digits or start or ends with an underscore.
+ digits or starts or ends with an underscore.
</para>
<para>
constants are discussed afterwards.
</para>
- <sect3>
+ <sect3 id="sql-syntax-strings">
<title>String Constants</title>
<para>
Transaction and command identifiers are 32 bit quantities.
</para>
- <para>
- A column can be referenced in the form:
-
-<synopsis>
-<replaceable>corelation</replaceable>.<replaceable>columnname</replaceable> `['<replaceable>subscript</replaceable>`]'
-</synopsis>
+ </sect1>
- <replaceable>corelation</replaceable> is either the name of a
- table, an alias for a table defined by means of a FROM clause, or
- the keyword <literal>NEW</literal> or <literal>OLD</literal>.
- (NEW and OLD can only appear in the action portion of a rule,
- while other corelation names can be used in any SQL statement.)
- The corelation name can be omitted if the column name is unique
- across all the tables being used in the current query. If
- <replaceable>column</replaceable> is of an array type, then the
- optional <replaceable>subscript</replaceable> selects a specific
- element in the array. If no subscript is provided, then the
- whole array is selected. Refer to the description of the
- particular commands in the <citetitle>PostgreSQL Reference
- Manual</citetitle> for the allowed syntax in each case.
- </para>
- </sect1>
+ <sect1 id="sql-expressions">
+ <title>Value Expressions</title>
- <sect1 id="sql-expressions">
- <title>Expressions</title>
+ <para>
+ Value expressions are used in a variety of syntactic contexts, such
+ as in the target list of the <command>SELECT</command> command, as
+ new column values in <command>INSERT</command> or
+ <command>UPDATE</command>, or in search conditions in a number of
+ commands. The result of a value expression is sometimes called a
+ <firstterm>scalar</firstterm>, to distinguish it from the result of
+ a table expression (which is a table). Value expressions are
+ therefore also called <firstterm>scalar expressions</firstterm> (or
+ even simply <firstterm>expressions</firstterm>). The expression
+ syntax allows the calculation of values from primitive parts using
+ arithmetic, logical, set, and other operations.
+ </para>
- <para>
- <acronym>SQL92</acronym> allows <firstterm>expressions</firstterm>
- to transform data in tables. Expressions may contain operators
- and functions.
- </para>
+ <para>
+ A value expression is one of the following:
- <para>
- An expression is one of the following:
+ <itemizedlist>
+ <listitem>
+ <para>
+ A constant or literal value; see <xref linkend="sql-syntax-constants">.
+ </para>
+ </listitem>
- <simplelist>
- <member>constant</member>
- <member>column</member>
- <member><replaceable>expression</replaceable> <replaceable>binary_operator</replaceable> <replaceable>expression</replaceable></member>
- <member><replaceable>expression</replaceable> <replaceable>right_unary_operator</replaceable></member>
- <member><replaceable>left_unary_operator</replaceable> <replaceable>expression</replaceable></member>
- <member>( <replaceable>expression</replaceable> )</member>
- <member>parameter</member>
- <member>functional expression</member>
- <member>aggregate expression</member>
- </simplelist>
- </para>
+ <listitem>
+ <para>
+ A column reference
+ </para>
+ </listitem>
- <para>
- We have already discussed constants and columns. The three kinds of
- operator expressions indicate respectively binary (infix), right-unary
- (suffix) and left-unary (prefix) operators. The following sections
- discuss the remaining options.
- </para>
+ <listitem>
+ <para>
+ An operator invocation:
+ <simplelist>
+ <member><replaceable>expression</replaceable> <replaceable>operator</replaceable> <replaceable>expression</replaceable> (binary infix operator)</member>
+ <member><replaceable>expression</replaceable> <replaceable>operator</replaceable> (unary postfix operator)</member>
+ <member><replaceable>operator</replaceable> <replaceable>expression</replaceable> (unary prefix operator)</member>
+ </simplelist>
+ where <replaceable>operator</replaceable> follows the syntax
+ rules of <xref linkend="sql-syntax-operators"> or is one of the
+ tokens <token>AND</token>, <token>OR</token>, and
+ <token>NOT</token>. What particular operators exist and whether
+ they are unary or binary depends on what operators have been
+ defined by the system or the user. <xref linkend="functions">
+ describes the built-in operators.
+ </para>
+ </listitem>
- <sect2>
- <title>Parameters</title>
+ <listitem>
+ <para>
+<synopsis>( <replaceable>expression</replaceable> )</synopsis>
+ Parentheses are used to group subexpressions and override precedence.
+ </para>
+ </listitem>
- <para>
- A <firstterm>parameter</firstterm>
- is used to indicate a parameter in a SQL function. Typically this
- is used in SQL function definition statements. The form of a
- parameter is:
-
- <synopsis>
-$<replaceable class="parameter">number</replaceable>
- </synopsis>
- </para>
+ <listitem>
+ <para>
+ A positional parameter reference, in the body of a function declaration.
+ </para>
+ </listitem>
- <para>
- For example, consider the definition of a function,
- <function>dept</function>, as
-
- <programlisting>
-CREATE FUNCTION dept (name)
- RETURNS dept
- AS 'select * from dept where name = $1'
- LANGUAGE 'sql';
- </programlisting>
- </para>
- </sect2>
+ <listitem>
+ <para>
+ A function call
+ </para>
+ </listitem>
- <sect2>
- <title>Functional Expressions</title>
+ <listitem>
+ <para>
+ An aggregate expression
+ </para>
+ </listitem>
- <para>
- A <firstterm>functional expression</firstterm>
- is the name of a legal SQL function, followed by its argument list
- enclosed in parentheses:
+ <listitem>
+ <para>
+ A scalar subquery. This is an ordinary
+ <command>SELECT</command> in parenthesis that returns exactly one
+ row with one column. It is an error to use a subquery that
+ returns more than one row or more than one column in the context
+ of a value expression.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
- <synopsis>
-<replaceable>function</replaceable> (<replaceable>expression</replaceable> [, <replaceable>expression</replaceable> ... ] )
- </synopsis>
- </para>
+ <para>
+ In addition to this list, there are a number of contructs that can
+ be classified as an expression but do not follow any general syntax
+ rules. These generally have the semantics of a function or
+ operator and are explained in the appropriate location in <xref
+ linkend="functions">. An example is the <literal>IS NULL</literal>
+ clause.
+ </para>
- <para>
- For example, the following computes the square root of an employee
- salary:
+ <para>
+ We have already discussed constants in <xref
+ linkend="sql-syntax-constants">. The following sections discuss
+ the remaining options.
+ </para>
- <programlisting>
-sqrt(emp.salary)
- </programlisting>
- </para>
- </sect2>
+ <sect2>
+ <title>Column References</title>
- <sect2 id="syntax-aggregates">
- <title>Aggregate Expressions</title>
+ <para>
+ A column can be referenced in the form:
+<synopsis>
+<replaceable>corelation</replaceable>.<replaceable>columnname</replaceable> `['<replaceable>subscript</replaceable>`]'
+</synopsis>
- <para>
- An <firstterm>aggregate expression</firstterm> represents the application
- of an aggregate function across the rows selected by a query.
- An aggregate function reduces multiple inputs to a single output value,
- such as the sum or average of the inputs.
- The syntax of an aggregate expression is one of the following:
+ <replaceable>corelation</replaceable> is either the name of a
+ table, an alias for a table defined by means of a FROM clause, or
+ the keyword <literal>NEW</literal> or <literal>OLD</literal>.
+ (NEW and OLD can only appear in the action portion of a rule,
+ while other corelation names can be used in any SQL statement.)
+ The corelation name can be omitted if the column name is unique
+ across all the tables being used in the current query. If
+ <replaceable>column</replaceable> is of an array type, then the
+ optional <replaceable>subscript</replaceable> selects a specific
+ element in the array. If no subscript is provided, then the whole
+ array is selected. Refer to the description of the particular
+ commands in the <citetitle>PostgreSQL Reference Manual</citetitle>
+ for the allowed syntax in each case.
+ </para>
+ </sect2>
- <simplelist>
- <member><replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)</member>
- <member><replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable>)</member>
- <member><replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)</member>
- <member><replaceable>aggregate_name</replaceable> ( * )</member>
- </simplelist>
+ <sect2>
+ <title>Positional Parameters</title>
- where <replaceable>aggregate_name</replaceable> is a previously defined
- aggregate, and <replaceable>expression</replaceable> is any expression
- that doesn't itself contain an aggregate expression.
- </para>
+ <para>
+ A positional parameter reference is used to indicate a parameter
+ in an SQL function. Typically this is used in SQL function
+ definition statements. The form of a parameter is:
+<synopsis>
+$<replaceable>number</replaceable>
+</synopsis>
+ </para>
- <para>
- The first form of aggregate expression invokes the aggregate across all
- input rows for which the given expression yields a non-null value.
- The second form is the same as the first, since ALL is the default.
- The third form invokes the aggregate for all distinct non-null values
- of the expression found in the input rows. The last form invokes the
- aggregate once for each input row regardless of null or non-null values;
- since no particular input value is specified, it is generally only useful
- for the count() aggregate.
- </para>
+ <para>
+ For example, consider the definition of a function,
+ <function>dept</function>, as
- <para>
- For example, count(*) yields the total number of input rows;
- count(f1) yields the number of input rows in which f1 is non-null;
- count(distinct f1) yields the number of distinct non-null values of f1.
- </para>
- </sect2>
+<programlisting>
+CREATE FUNCTION dept (text) RETURNS dept
+ AS 'select * from dept where name = $1'
+ LANGUAGE 'sql';
+</programlisting>
- <sect2>
- <title>Target List</title>
+ Here the <literal>$1</literal> will be replaced by the first
+ function argument when the function is invoked.
+ </para>
+ </sect2>
- <para>
- A <firstterm>target list</firstterm>
- is a comma-separated list of one or more elements, each
- of which must be of the form:
-
- <synopsis>
-<replaceable>expression</replaceable> [ AS <replaceable>result_attname</replaceable> ]
- </synopsis>
-
- where <replaceable>result_attname</replaceable>
- is the name to be assigned to the created column. If
- <replaceable>result_attname</replaceable>
- is not present, then <productname>Postgres</productname> selects a
- default name based on the contents of <replaceable>expression</replaceable>.
- If <replaceable>expression</replaceable> is a simple attribute reference
- then the default name will be the same as that attribute's name, but
- otherwise the implementation is free to assign any default name.
- </para>
- </sect2>
+ <sect2>
+ <title>Function Calls</title>
- <sect2>
- <title>Qualification</title>
+ <para>
+ The syntax for a function call is the name of a legal function
+ (subject to the syntax rules for identifiers of <xref
+ linkend="sql-syntax-identifiers"> , followed by its argument list
+ enclosed in parentheses:
- <para>
- A <firstterm>qualification</firstterm>
- consists of any number of clauses connected by the logical operators:
+<synopsis>
+<replaceable>function</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional> )
+</synopsis>
+ </para>
- <simplelist>
- <member>NOT</member>
- <member>AND</member>
- <member>OR</member>
- </simplelist>
+ <para>
+ For example, the following computes the square root of 2:
+<programlisting>
+sqrt(2)
+</programlisting>
+ </para>
- A clause is an <replaceable>expression</replaceable>
- that evaluates to a <literal>boolean</literal> over a set of instances.
- </para>
- </sect2>
+ <para>
+ The list of built-in functions is in <xref linkend="functions">.
+ Other functions may be added by the user.
+ </para>
+ </sect2>
- <sect2>
- <title>From List</title>
+ <sect2 id="syntax-aggregates">
+ <title>Aggregate Expressions</title>
- <para>
- The <firstterm>from list</firstterm>
- is a comma-separated list of <firstterm>from-expressions</firstterm>.
- The simplest possibility for a from-expression is:
-
- <synopsis>
-<replaceable>table_reference</replaceable> [ [ AS ] <replaceable class="PARAMETER">alias</replaceable> ]
- </synopsis>
-
- where <replaceable>table_reference</replaceable> is of the form
-
- <synopsis>
-[ ONLY ] <replaceable class="PARAMETER">table_name</replaceable> [ * ]
- </synopsis>
-
- The from-expression defines an instance variable that ranges over the
- rows of the specified table. The instance variable's name is either
- the table name, or the <replaceable>alias</replaceable> if one is given.
- Ordinarily, if the table has child tables then the instance variable
- will range over all rows in the inheritance hierarchy starting with
- the specified table. If <literal>ONLY</literal> is specified then
- child tables are not included. A trailing asterisk <literal>*</literal>
- can be written to specifically indicate that child tables are included
- (<literal>ONLY</literal> and <literal>*</literal> are mutually
- exclusive).
- </para>
+ <para>
+ An <firstterm>aggregate expression</firstterm> represents the
+ application of an aggregate function across the rows selected by a
+ query. An aggregate function reduces multiple inputs to a single
+ output value, such as the sum or average of the inputs. The
+ syntax of an aggregate expression is one of the following:
- <para>
- A from-expression can also be a sub-query:
+ <simplelist>
+ <member><replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)</member>
+ <member><replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable>)</member>
+ <member><replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)</member>
+ <member><replaceable>aggregate_name</replaceable> ( * )</member>
+ </simplelist>
- <synopsis>
-( <replaceable class="PARAMETER">select-statement</replaceable> ) [ AS ] <replaceable class="PARAMETER">alias</replaceable>
- </synopsis>
+ where <replaceable>aggregate_name</replaceable> is a previously
+ defined aggregate, and <replaceable>expression</replaceable> is
+ any expression that does not itself contain an aggregate
+ expression.
+ </para>
- Here, the effect is as though the SELECT were executed and its results
- stored in a temporary table, which then becomes available as an instance
- variable under the given <replaceable>alias</replaceable>.
- </para>
+ <para>
+ The first form of aggregate expression invokes the aggregate
+ across all input rows for which the given expression yields a
+ non-NULL value. The second form is the same as the first, since
+ <literal>ALL</literal> is the default. The third form invokes the
+ aggregate for all distinct non-NULL values of the expression found
+ in the input rows. The last form invokes the aggregate once for
+ each input row regardless of NULL or non-NULL values; since no
+ particular input value is specified, it is generally only useful
+ for the <function>count()</function> aggregate function.
+ </para>
- <para>
- Finally, a from-expression can be built up from simpler from-expressions
- using JOIN clauses:
+ <para>
+ For example, <literal>count(*)</literal> yields the total number
+ of input rows; <literal>count(f1)</literal> yields the number of
+ input rows in which <literal>f1</literal> is non-NULL;
+ <literal>count(distinct f1)</literal> yields the number of
+ distinct non-NULL values of <literal>f1</literal>.
+ </para>
- <synopsis>
-<replaceable class="PARAMETER">from_expression</replaceable> [ NATURAL ] <replaceable class="PARAMETER">join_type</replaceable> <replaceable class="PARAMETER">from_expression</replaceable>
- [ ON <replaceable class="PARAMETER">join_condition</replaceable> | USING ( <replaceable class="PARAMETER">join_column_list</replaceable> ) ]
- </synopsis>
+ <para>
+ The predefined aggregate functions are described in <xref
+ linkend="functions-aggregate">.
+ </para>
+ </sect2>
- This syntax allows specification of <firstterm>outer joins</firstterm>.
- For details see the reference page for SELECT.
- </para>
- </sect2>
+ </sect1>
- <sect2 id="sql-precedence">
+ <sect1 id="sql-precedence">
<title>Lexical Precedence</title>
<para>
</para>
<table tocentry="1">
- <title>Operator Ordering (decreasing precedence)</title>
+ <title>Operator Precedence (decreasing)</title>
<tgroup cols="2">
<thead>
the same precedence as the built-in <quote>+</quote> operator, no
matter what yours does.
</para>
+ </sect1>
+
+
+ <sect1 id="sql-table-expressions">
+ <title>Table Expressions</title>
+
+ <para>
+ A <firstterm>table expression</firstterm> specifies a table. The
+ table expression contains a FROM clause that is optionally followed
+ by WHERE, GROUP BY, and HAVING clauses. Trivial table expressions
+ simply refer to a table on disk, a so-called base table, but more
+ complex expressions can be used to modify or combine base tables in
+ various ways.
+ </para>
+
+ <para>
+ The general syntax of the <command>SELECT</command> command is
+<synopsis>
+SELECT <replaceable>select_list</replaceable> <replaceable>table_expression</replaceable>
+</synopsis>
+
+ The <replaceable>select_list</replaceable> is a comma separated
+ list of <replaceable>value expressions</replaceable> as defined in
+ <xref linkend="sql-expressions"> that specify the derived columns
+ of the query output table. Column names in the derived table that
+ is the result of the <replaceable>table_expression</replaceable>
+ can be used in the <replaceable>value expression</replaceable>s of
+ the <replaceable>select_list</replaceable>.
+ </para>
+
+ <para>
+ The WHERE, GROUP BY, and HAVING clauses in the table expression
+ specify a pipeline of successive transformations performed on the
+ table derived in the FROM clause. The final transformed table that
+ is derived provides the input rows used to derive output rows as
+ specified by the select list of derived column value expressions.
+ </para>
+
+ <sect2>
+ <title>FROM clause</title>
+
+ <para>
+ The FROM clause derives a table from one or more other tables
+ given in a comma-separated table reference list.
+<synopsis>
+FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_reference</replaceable> <optional>, ...</optional></optional>
+</synopsis>
+
+ A table reference may be a table name or a derived table such as a
+ subquery, a table join, or complex combinations of these. If more
+ than one table reference is listed in the FROM clause they are
+ CROSS JOINed (see below) to form the derived table that may then
+ be subject to transformations by the WHERE, GROUP BY, and HAVING
+ clauses and is finally the result of the overall table expression.
+ </para>
+
+ <para>
+ If a table reference is a simple table name and it is the
+ supertable in a table inheritance hierarchy, rows of the table
+ include rows from all of its subtable successors unless the
+ keyword ONLY precedes the table name.
+ </para>
+
+ <sect3>
+ <title>Joined Tables</title>
+
+ <para>
+ A joined table is a table derived from two other (real or
+ derived) tables according to the rules of the particular join
+ type. INNER, OUTER, NATURAL, and CROSS JOIN are supported.
+ </para>
+
+ <variablelist>
+ <title>Join Types</title>
+
+ <varlistentry>
+ <term>CROSS JOIN</term>
+
+ <listitem>
+<synopsis>
+<replaceable>T1</replaceable> CROSS JOIN <replaceable>T2</replaceable>
+</synopsis>
+
+ <para>
+ For each combination of rows from
+ <replaceable>T1</replaceable> and
+ <replaceable>T2</replaceable> the derived table will contain a
+ row consisting of all columns in <replaceable>T1</replaceable>
+ followed by all columns in <replaceable>T2</replaceable>. If
+ the tables have have N and M rows respectively, the joined
+ table will have N * M rows. A cross join is essentially an
+ <literal>INNER JOIN ON TRUE</literal>.
+ </para>
+
+ <tip>
+ <para>
+ <literal>FROM <replaceable>T1</replaceable> CROSS JOIN
+ <replaceable>T2</replaceable></literal> is equivalent to
+ <literal>FROM <replaceable>T1</replaceable>,
+ <replaceable>T2</replaceable></literal>.
+ </para>
+ </tip>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>Qualified JOINs</term>
+ <listitem>
+
+<synopsis>
+<replaceable>T1</replaceable> { <optional>INNER</optional> | { LEFT | RIGHT | FULL } <optional>OUTER</optional> } JOIN <replaceable>T2</replaceable> ON <replaceable>boolean expression</replaceable>
+<replaceable>T1</replaceable> { <optional>INNER</optional> | { LEFT | RIGHT | FULL } <optional>OUTER</optional> } JOIN <replaceable>T2</replaceable> USING ( <replaceable>join column list</replaceable> )
+</synopsis>
+
+ <para>
+ The words <token>INNER</token> and <token>OUTER</token> are
+ optional for all JOINs. <token>INNER</token> is the default;
+ <token>LEFT</token>, <token>RIGHT</token>, and
+ <token>FULL</token> are for OUTER JOINs only.
+ </para>
+
+ <para>
+ The <firstterm>join condition</firstterm> is specified in the
+ ON or USING clause. (The meaning of the join condition
+ depends on the particular join type; see below.) The ON
+ clause takes a boolean value expression of the same kind as is
+ used in a WHERE clause. The USING clause takes a
+ comma-separated list of column names, which the joined tables
+ must have in common, and joins the tables on the equality of
+ those columns as a set, resulting in a joined table having one
+ column for each common column listed and all of the other
+ columns from both tables. 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 ON is used there will be two columns a, b, and c in the
+ result, whereas with USING there will be only one of each.
+ </para>
+
+ <variablelist>
+ <varlistentry>
+ <term>INNER JOIN</term>
+
+ <listitem>
+ <para>
+ For each row R1 of T1, the joined table has a row for each
+ row in T2 that satisfies the join condition with R1.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>LEFT OUTER JOIN</term>
+
+ <listitem>
+ <para>
+ First, an INNER JOIN is performed. Then, for a row in T1
+ that does not satisfy the join condition with any row in
+ T2, a joined row is returned with NULL values in columns of
+ T2. Thus, the joined table unconditionally has a row for each
+ row in T1.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>RIGHT OUTER JOIN</term>
+
+ <listitem>
+ <para>
+ This is like a left join, only that the result table will
+ unconditionally have a row for each row in T2.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>FULL OUTER JOIN</term>
+
+ <listitem>
+ <para>
+ First, an INNER JOIN is performed. Then, for each row in
+ T1 that does not satisfy the join condition with any row in
+ T2, a joined row is returned with null values in columns of
+ T2. Also, for each row of T2 that does not satisfy the
+ join condition with any row in T1, a joined row with null
+ values in the columns of T1 is returned.
+ </para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>NATURAL JOIN</term>
+
+ <listitem>
+<synopsis>
+<replaceable>T1</replaceable> NATURAL { <optional>INNER</optional> | { LEFT | RIGHT | FULL } <optional>OUTER</optional> JOIN <replaceable>T2</replaceable>
+</synopsis>
+ <para>
+ A natural join creates a joined table where every pair of matching
+ column names between the two tables are merged into one column. The
+ join specification is effectively a USING clause containing all the
+ common column names and is otherwise like a Qualified JOIN.
+ </para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ <para>
+ Joins of all types can be chained together or nested where either
+ or both of <replaceable>T1</replaceable> and
+ <replaceable>T2</replaceable> may be JOINed tables. Parenthesis
+ can be used around JOIN clauses to control the join order which
+ are otherwise left to right.
+ </para>
+ </sect3>
+
+ <sect3 id="sql-subqueries">
+ <title>Subqueries</title>
+
+ <para>
+ Subqueries specifying a derived table must be enclosed in
+ parenthesis and <emphasis>must</emphasis> be named using an AS
+ clause. (See <xref linkend="sql-table-aliases">.)
+ </para>
+
+<programlisting>
+FROM (SELECT * FROM table1) AS alias_name
+</programlisting>
+
+ <para>
+ This example is equivalent to <literal>FROM table1 AS
+ alias_name</literal>. Many subquieries can be written as table
+ joins instead.
+ </para>
+ </sect3>
+
+ <sect3 id="sql-table-aliases">
+ <title>Table and Column Aliases</title>
+
+ <para>
+ A temporary name can be given to tables and complex table
+ references to be used for references to the derived table in
+ further processing. This is called a <firstterm>table
+ alias</firstterm>.
+<synopsis>
+FROM <replaceable>table_reference</replaceable> AS <replaceable>alias</replaceable>
+</synopsis>
+ Here, <replaceable>alias</replaceable> can be any regular
+ identifier. The alias becomes the new name of the table
+ reference for the current query -- it is no longer possible to
+ refer to the table by the original name (if the table reference
+ was an ordinary base table). Thus
+<programlisting>
+SELECT * FROM my_table AS m WHERE my_table.a > 5;
+</programlisting>
+ is not valid SQL syntax. What will happen instead, as a
+ <productname>Postgres</productname> extension, is that an implict
+ table reference is added to the FROM clause, so the query is
+ processed as if it was written as
+<programlisting>
+SELECT * FROM my_table AS m, my_table WHERE my_table.a > 5;
+</programlisting>
+ 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 ...
+</programlisting>
+ Additionally, an alias is required if the table reference is a
+ subquery.
+ </para>
+
+ <para>
+ Parenthesis 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:
+<programlisting>
+SELECT * FROM (my_table AS a CROSS JOIN my_table) AS b ...
+</programlisting>
+ </para>
+
+ <para>
+<synopsis>
+FROM <replaceable>table_reference</replaceable> <replaceable>alias</replaceable>
+</synopsis>
+ This form is equivalent the previously treated one; the
+ <token>AS</token> key word is noise.
+ </para>
+
+ <para>
+<synopsis>
+FROM <replaceable>table_reference</replaceable> <optional>AS</optional> <replaceable>alias</replaceable> ( <replaceable>column1</replaceable> <optional>, <replaceable>column2</replaceable> <optional>, ...</optional></optional> )
+</synopsis>
+ In addition to renaming the table as described above, the columns
+ of the table are also given temporary names. If less column
+ aliases are specified than the actual table has columns, the last
+ columns are not renamed. This syntax is especially useful for
+ self-joins or subqueries.
+ </para>
+ </sect3>
+
+ <sect3>
+ <title>Examples</title>
+
+ <para>
+<programlisting>
+FROM T1 INNER JOIN T2 USING (C)
+FROM T1 LEFT OUTER JOIN T2 USING (C)
+FROM (T1 RIGHT OUTER JOIN T2 ON (T1C1=T2C1)) AS DT1
+FROM (T1 FULL OUTER JOIN T2 USING (C)) AS DT1 (DT1C1, DT1C2)
+
+FROM T1 NATURAL INNER JOIN T2
+FROM T1 NATURAL LEFT OUTER JOIN T2
+FROM T1 NATURAL RIGHT OUTER JOIN T2
+FROM T1 NATURAL FULL OUTER JOIN T2
+
+FROM (SELECT * FROM T1) DT1 CROSS JOIN T2, T3
+FROM (SELECT * FROM T1) DT1, T2, T3
+</programlisting>
+
+ Above are some examples of joined tables and complex derived
+ tables. Notice how the AS clause renames or names a derived
+ table and how the optional comma-separated list of column names
+ that follows gives names or renames the columns. The last two
+ FROM clauses produce the same derived table from T1, T2, and T3.
+ The AS keyword was omitted in naming the subquery as DT1. The
+ keywords OUTER and INNER are noise that can be omitted also.
+ </para>
+ </sect3>
+
+ </sect2>
+
+ <sect2>
+ <title>WHERE clause</title>
+
+ <para>
+ The syntax of the WHERE clause is
+<synopsis>
+WHERE <replaceable>search condition</replaceable>
+</synopsis>
+ where <replaceable>search condition</replaceable> is any value
+ expression as defined in <xref linkend="sql-expressions"> that
+ returns a value of type <type>boolean</type>.
+ </para>
+
+ <para>
+ After the processing of the FROM clause is done, each row of the
+ derived table is checked against the search condition. If the
+ result of the condition is true, the row is kept in the output
+ table, otherwise (that is, if the result is false or NULL) it is
+ discared. The search condition typically references at least some
+ column in the table generated in the FROM clause; this is not
+ required, but otherwise the WHERE clause will be fairly useless.
+ </para>
+
+ <note>
+ <para>
+ Before the implementation of the JOIN syntax, it was necessary to
+ put the join condition of an inner join in the WHERE clause. For
+ example, these table expressions are equivalent:
+<programlisting>
+FROM a, b WHERE a.id = b.id AND b.val > 5
+</programlisting>
+ and
+<programlisting>
+FROM a INNER JOIN b ON (a.id = b.id) WHERE b.val > 5
+</programlisting>
+ or perhaps even
+<programlisting>
+FROM a NATURAL JOIN b WHERE b.val > 5
+</programlisting>
+ Which one of these you use is mainly a matter of style. The JOIN
+ syntax in the FROM clause is probably not as portable to other
+ products. For outer joins there is no choice in any case: they
+ must be done in the FROM clause.
+ </para>
+ </note>
+
+<programlisting>
+FROM FDT WHERE
+ C1 > 5
+
+FROM FDT WHERE
+ C1 IN (1, 2, 3)
+FROM FDT WHERE
+ C1 IN (SELECT C1 FROM T2)
+FROM FDT WHERE
+ C1 IN (SELECT C3 FROM T2 WHERE C2 = FDT.C1 + 10)
+
+FROM FDT WHERE
+ C1 BETWEEN (SELECT C3 FROM T2 WHERE C2 = FDT.C1 + 10) AND 100
+
+FROM FDT WHERE
+ EXISTS (SELECT C1 FROM T2 WHERE C2 > FDT.C1)
+</programlisting>
+
+ <para>
+ In the examples above, FDT is the table derived in the FROM
+ clause. Rows that do not meet the search condition of the where
+ clause are eliminated from FDT. Notice the use of scalar
+ subqueries as value expressions (C2 assumed UNIQUE). Just like
+ any other query, the subqueries can employ complex table
+ expressions. Notice how FDT is referenced in the subqueries.
+ Qualifying C1 as FDT.C1 is only necessary if C1 is the name of a
+ column in the derived input table of the subquery. Qualifying the
+ column name adds clarity even when it is not needed. The column
+ naming scope of an outer query extends into its inner queries.
+ </para>
</sect2>
+<!-- This is confusing as heck. Make it simpler. -->
+
+<![IGNORE[
+
+ <sect2>
+ <title>GROUP BY and HAVING clauses</title>
+
+ <para>
+ After passing the WHERE filter, the derived input table may be
+ subject to grouping, using the GROUP BY clause, and elimination of
+ group rows using the HAVING clause. (The HAVING clause can also
+ be used without GROUP BY, but then it is equivalent to the WHERE
+ clause.)
+ </para>
+
+ <para>
+ In standard SQL, the GROUP BY clause takes a list of column names,
+ that specify a subrow, from the derived input table produced by
+ the previous WHERE or FROM clause and partitions the table into
+ groups with duplicate subrows such that within a column of the
+ subrow, no column value is distinct from other column values. The
+ resulting derived input table is a special type of table, called a
+ grouped table, which still contains all columns but only
+ references to columns of the grouped subrow, and group aggregates,
+ derived from any of the columns, may appear in derived column
+ value expressions in the query select list. When deriving an
+ output table from a query using a grouped input table, each output
+ row is derived from a corresponding group/partition of the grouped
+ table. Aggregates computed in a derived output column are
+ aggregates on the current partition/group of the grouped input
+ table being processed. Only one output table row results per
+ group/partition of the grouped input table.
+ </para>
+
+ <para>
+ Postgres has extended the GROUP BY clause to allow some
+ non-standard, but useful behavior. Derived output columns, given
+ names using an AS clause in the query select list, may appear in
+ the GROUP BY clause in combination with, or instead of, the input
+ table column names. Tables may also be grouped by arbitrary
+ expressions. If output table column names appear in the GROUP BY
+ list, then the input table is augmented with additional columns of
+ the output table columns listed in the GROUP BY clause. The value
+ for each row in the additional columns is computed from the value
+ expression that defines the output column in the query select
+ list. The augmented input table is grouped by the column names
+ listed in the GROUP BY clause. The resulting grouped augmented
+ input table is then treated according standard SQL GROUP BY
+ semantics. Only the columns of the unaugmented input table in the
+ grouped subrow (if any), and group aggregates, derived from any of
+ the columns of the unaugmented input table, may be referenced in
+ the value expressions of the derived output columns of the
+ query. Output columns derived with an aggregate expression cannot
+ be named in the GROUP BY clause.
+ </para>
+
+ <para>
+ A HAVING clause may optionally follow a GROUP BY clause. The
+ HAVING clause selects or eliminates, depending on which
+ perspective is taken, groups from the grouped table derived in the
+ GROUP BY clause that precedes it. The search condition is the
+ same type of expression allowed in a WHERE clause and may
+ reference any of the input table column names in the grouped
+ subrow, but may not reference any others or any named output
+ columns. When the search condition results in TRUE the group is
+ retained, otherwise the group is eliminated.
+ </para>
+ </sect2>
+
+ <sect2>
+ <title>ORDER BY and LIMIT clauses</title>
+
+ <para>
+ ORDER BY and LIMIT clauses are not clauses of a table expression.
+ They are optional clauses that may follow a query expression and
+ are discussed here because they are commonly used with the
+ clauses above.
+ </para>
+
+ <para>
+ ORDER BY takes a comma-separated list of columns and performs a
+ cascaded ordering of the table by the columns listed, in the
+ order listed. The keyword DESC or ASC may follow any column name
+ or expression in the list to specify descending or ascending
+ ordering, respectively. Ascending order is the default. The
+ ORDER BY clause conforms to the SQL standard but is extended in
+ Postgres. Postgres allows ORDER BY to reference both output
+ table columns, as named in the select list using the AS clause,
+ and input table columns, as given by the table derived in the
+ FROM clause and other previous clauses. Postgres also extends
+ ORDER BY to allow ordering by arbitrary expressions. If used in a
+ query with a GROUP BY clause, the ORDER BY clause can only
+ reference output table column names and grouped input table
+ columns.
+ </para>
+
+ <para>
+ LIMIT is not a standard SQL clause. LIMIT is a Postgres
+ extension that limits the number of rows that will be returned
+ from a query. The rows returned by a query using the LIMIT
+ clause are random if no ORDER BY clause is specified. A LIMIT
+ clause may optionally be followed by an OFFSET clause which
+ specifies a number of rows to be skipped in the output table
+ before returning the number of rows specified in the LIMIT
+ clause.
+ </para>
+ </sect2>
+]]>
</sect1>
- </chapter>
+
+</chapter>
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