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<chapter id="sql">
named attributes have to be equal). Finally we project out all
columns but S.SNAME and P.PNAME.
</para>
+
+ <para>
+ Another way to perform joins is to use the SQL JOIN syntax as follows:
+ <programlisting>
+select sname, pname from supplier
+ JOIN sells USING (sno)
+ JOIN part USING (pno);
+ </programlisting>
+ giving again:
+ <programlisting>
+ sname | pname
+-------+-------
+ Smith | Screw
+ Adams | Screw
+ Smith | Nut
+ Blake | Nut
+ Adams | Bolt
+ Blake | Bolt
+ Jones | Cam
+ Blake | Cam
+(8 rows)
+ </programlisting>
+ </para>
+
+ <para>
+ A joined table, created using JOIN syntax, is a table reference list
+ item that occurs in a FROM clause and before any WHERE, GROUP BY,
+ or HAVING clause. Other table references, including table names or
+ other JOIN clauses, may be included in the FROM clause if separated
+ by commas. A JOIN of two tables is logically like any other
+ table listed in the FROM clause. A JOINed table can only be JOINed
+ to additional tables in a Qualified JOIN as indicated by the
+ elipses below.
+ </para>
+
+ <variablelist>
+ <title>Join Types</title>
+ <varlistentry>
+ <term>CROSS JOIN</term>
+ <listitem>
+ <cmdsynopsis>
+ <arg choice="req"> <replaceable class="parameter">T1</replaceable> </arg>
+ <arg choice="plain">CROSS</arg>
+ <command> JOIN </command>
+ <arg choice="req"> <replaceable class="parameter">T2</replaceable> </arg>
+ </cmdsynopsis>
+
+ <para>
+ A cross join takes two tables T1 and T2 having N and M rows
+ respectively, and returns a joined table containing a cross
+ product, NxM, of joined rows. For each row R1 of T1, each row
+ R2 of T2 is joined with R1 to yield a joined table row JR
+ consisting of all fields in R1 and R2.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>Qualified JOINs</term>
+ <listitem>
+ <cmdsynopsis>
+ <arg choice="req"> <replaceable class="parameter">T1</replaceable> </arg>
+ <arg>
+ <group>
+ <arg choice="plain"> INNER </arg>
+ <arg>
+ <group>
+ <arg> LEFT </arg>
+ <arg> RIGHT </arg>
+ <arg> FULL </arg>
+ </group>
+ <arg> OUTER </arg>
+ </arg>
+ </group>
+ </arg>
+ <command> JOIN </command>
+ <arg choice="req"> <replaceable class="parameter">T2</replaceable> </arg>
+ <arg choice="req">
+ <group>
+ <arg> ON <replaceable>search condition</replaceable></arg>
+ <arg> USING ( <replaceable>join column list</replaceable> ) </arg>
+ </group>
+ </arg>
+ <arg choice="plain"> ... </arg>
+ </cmdsynopsis>
+
+ <para>
+ Only the qualified JOIN types can use ON or USING clauses. The ON clause
+ takes a <replaceable>search condition</replaceable>, which is the same
+ as 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 those columns, resulting in a joined table having one
+ column for each common column and all of the other columns from both tables.
+ Like all SELECT queries, the <replaceable>select list</replaceable> of the
+ SELECT query, before the FROM clause, decides which columns from the joined
+ table are in the result table returned.
+ </para>
+
+ <!-- begin join semantics -->
+ <variablelist>
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <arg> INNER </arg>
+ <command> JOIN </command>
+ </cmdsynopsis>
+ </term>
+ <listitem>
+ <para>
+ For each row R1 of T1, the joined table has a row for each row
+ in T2 that satisfies the join specification with R1.
+ </para>
+ <tip>
+ <para>
+ The words INNER and OUTER are optional for all JOINs.
+ INNER is the default. LEFT, RIGHT, and FULL are for
+ OUTER JOINs only.
+ </para>
+ </tip>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <arg choice="plain"> LEFT </arg>
+ <arg> OUTER </arg>
+ <command> JOIN </command>
+ </cmdsynopsis>
+ </term>
+ <listitem>
+ <para>
+ First, an INNER JOIN is performed.
+ Then, where a row in T1 does not satisfy the join specification
+ with any row in T2, a joined row is returned with null fields in
+ columns from T2.
+ </para>
+ <tip>
+ <para>
+ The joined table unconditionally has a row for each row in T1.
+ </para>
+ </tip>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <arg choice="plain"> RIGHT </arg>
+ <arg> OUTER </arg>
+ <command> JOIN </command>
+ </cmdsynopsis>
+ </term>
+ <listitem>
+ <para>
+ Rule 1: For each row R2 of T2, the joined table has a row for each
+ row in T1 that satisfies the join specification with R2 (transposed
+ [INNER] JOIN).
+ Rule 2: Where a row in T2 does not satisfy the join specification
+ with any row in T1, a joined row is returned with null fields in
+ columns from T1.
+ </para>
+ <tip>
+ <para>
+ The joined table unconditionally has a row for each row in T2.
+ </para>
+ </tip>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <arg choice="plain"> FULL </arg>
+ <arg> OUTER </arg>
+ <command> JOIN </command>
+ </cmdsynopsis>
+ </term>
+ <listitem>
+ <para>
+ First, a LEFT [OUTER] JOIN is performed.
+ Then, Rule 2 of a RIGHT [OUTER] JOIN is performed.
+ </para>
+ <tip>
+ <para>
+ The joined table unconditionally has a row for every row of T1
+ and a row for every row of T2.
+ </para>
+ </tip>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <!-- end join semantics -->
+
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>NATURAL JOINs</term>
+ <listitem>
+ <cmdsynopsis>
+ <arg choice="req"> <replaceable class="parameter">T1</replaceable> </arg>
+ <arg>
+ <arg choice="plain"> NATURAL </arg>
+ <group>
+ <arg choice="plain"> INNER </arg>
+ <arg>
+ <group>
+ <arg> LEFT </arg>
+ <arg> RIGHT </arg>
+ <arg> FULL </arg>
+ </group>
+ <arg> OUTER </arg>
+ </arg>
+ </group>
+ </arg>
+ <command> JOIN </command>
+ <arg choice="req"> <replaceable class="parameter">T2</replaceable> </arg>
+ </cmdsynopsis>
+
+ <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 except
+ additional JOINs to the JOINed table are not permitted.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>UNION JOIN</term>
+ <listitem><para>Deprecated.</para></listitem>
+ </varlistentry>
+ </variablelist>
+
</sect3>
<sect3>
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