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Improve documentation of JOIN syntax. Explain NATURAL as an alternative

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to ON and USING for specifying the join condition, not as an independent
kind of join semantics.
This commit is contained in:
Tom Lane
2001-02-15 04:10:54 +00:00
parent 5c90733558
commit 52cacf46b4
2 changed files with 138 additions and 132 deletions
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162
doc/src/sgml/sql.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/sql.sgml,v 1.18 2001/01/27 05:07:28 momjian Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/sql.sgml,v 1.19 2001/02/15 04:10:54 tgl Exp $
-->
<chapter id="sql">
@ -1063,15 +1063,11 @@ select sname, pname from supplier
</para>
<para>
JOINs of all types can be chained together or nested where either or both of
<replaceable class="parameter">T1</replaceable> and
<replaceable class="parameter">T2</replaceable> may be JOINed tables.
A Qualified JOIN may be JOINed to another table (or JOINed table)
following its join specification, which consists of either an
ON <replaceable>search condition</replaceable> or
USING ( <replaceable>join column list</replaceable> ) clause.
Parenthesis can be used around JOIN clauses to control the order
of JOINs which are otherwise processed left to right.
SQL JOINs come in two main types, CROSS JOINs (unqualified joins)
and <firstterm>qualified JOINs</>. Qualified joins can be further
subdivided based on the way in which the <firstterm>join condition</>
is specified (ON, USING, or NATURAL) and the way in which it is
applied (INNER or OUTER join).
</para>
<variablelist>
@ -1081,18 +1077,17 @@ select sname, pname from supplier
<listitem>
<cmdsynopsis>
<arg choice="req"> <replaceable class="parameter">T1</replaceable> </arg>
<arg choice="plain">CROSS</arg>
<command> JOIN </command>
<command> CROSS 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
respectively, and returns a joined table containing all
N*M possible 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. A CROSS JOIN is
essentially an INNER JOIN ON TRUE.
equivalent to an INNER JOIN ON TRUE.
</para>
</listitem>
</varlistentry>
@ -1102,34 +1097,41 @@ select sname, pname from supplier
<listitem>
<cmdsynopsis>
<arg choice="req"> <replaceable class="parameter">T1</replaceable> </arg>
<group choice="opt">
<arg choice="opt"> INNER </arg>
<arg>
<group choice="req">
<arg choice="plain"> LEFT </arg>
<arg choice="plain"> RIGHT </arg>
<arg choice="plain"> FULL </arg>
</group>
<arg choice="opt"> OUTER </arg>
<arg choice="req"> <replaceable class="parameter">T1</replaceable> </arg>
<arg choice="opt"> NATURAL </arg>
<group choice="opt">
<arg choice="opt"> INNER </arg>
<arg>
<group choice="req">
<arg choice="plain"> LEFT </arg>
<arg choice="plain"> RIGHT </arg>
<arg choice="plain"> FULL </arg>
</group>
<arg choice="opt"> OUTER </arg>
</arg>
</group>
<command> JOIN </command>
<arg choice="req"> <replaceable class="parameter">T2</replaceable> </arg>
<group choice="req">
<arg> ON <replaceable>search condition</replaceable></arg>
<arg> USING ( <replaceable>join column list</replaceable> ) </arg>
</group>
<arg choice="plain"> ... </arg>
<command> JOIN </command>
<arg choice="req"> <replaceable class="parameter">T2</replaceable> </arg>
<group choice="req">
<arg> ON <replaceable>search condition</replaceable></arg>
<arg> USING ( <replaceable>join column list</replaceable> ) </arg>
</group>
</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.
A qualified JOIN must specify its join condition
by providing one (and only one) of NATURAL, ON, or
USING. 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 equality of those columns. NATURAL is
shorthand for a USING clause that lists all the common
column names of the two tables. A side-effect of both
USING and NATURAL is that only one copy of each joined
column is emitted into the result table (compare the
relational-algebra definition of JOIN, shown earlier).
</para>
<!-- begin join semantics -->
@ -1144,13 +1146,13 @@ select sname, pname from supplier
<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.
in T2 that satisfies the join condition with R1.
</para>
<tip>
<para>
<para>
The words INNER and OUTER are optional for all JOINs.
INNER is the default. LEFT, RIGHT, and FULL are for
OUTER JOINs only.
INNER is the default. LEFT, RIGHT, and FULL imply an
OUTER JOIN.
</para>
</tip>
</listitem>
@ -1166,9 +1168,9 @@ select sname, pname from supplier
<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.
Then, for each row in T1 that does not satisfy the join
condition with any row in T2, an additional joined row is
returned with null fields in the columns from T2.
</para>
<tip>
<para>
@ -1187,12 +1189,10 @@ select sname, pname from supplier
</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.
First, an INNER JOIN is performed.
Then, for each row in T2 that does not satisfy the join
condition with any row in T1, an additional joined row is
returned with null fields in the columns from T1.
</para>
<tip>
<para>
@ -1211,8 +1211,13 @@ select sname, pname from supplier
</term>
<listitem>
<para>
First, a LEFT [OUTER] JOIN is performed.
Then, Rule 2 of a RIGHT [OUTER] JOIN is performed.
First, an INNER JOIN is performed.
Then, for each row in T1 that does not satisfy the join
condition with any row in T2, an additional joined row is
returned with null fields in the columns from T2.
Also, for each row in T2 that does not satisfy the join
condition with any row in T1, an additional joined row is
returned with null fields in the columns from T1.
</para>
<tip>
<para>
@ -1227,40 +1232,15 @@ select sname, pname from supplier
</listitem>
</varlistentry>
<varlistentry>
<term>NATURAL JOINs</term>
<listitem>
<cmdsynopsis>
<arg choice="req"> <replaceable class="parameter">T1</replaceable> </arg>
<arg choice="plain"> NATURAL </arg>
<group choice="opt">
<arg choice="opt"> INNER </arg>
<arg>
<group choice="req">
<arg choice="plain"> LEFT </arg>
<arg choice="plain"> RIGHT </arg>
<arg choice="plain"> FULL </arg>
</group>
<arg choice="opt"> OUTER </arg>
</arg>
</group>
<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.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
JOINs of all types can be chained together or nested where either or both of
<replaceable class="parameter">T1</replaceable> and
<replaceable class="parameter">T2</replaceable> may be JOINed tables.
Parenthesis can be used around JOIN clauses to control the order
of JOINs which are otherwise processed left to right.
</para>
</sect3>
@ -1302,7 +1282,7 @@ SELECT AVG(PRICE) AS AVG_PRICE
</para>
<para>
If we want to know how many parts are stored in table PART we use
If we want to know how many parts are defined in table PART we use
the statement:
<programlisting>
@ -1332,7 +1312,7 @@ SELECT COUNT(PNO)
aggregate operators described above can be applied to the groups ---
i.e. the value of the aggregate operator is no longer calculated over
all the values of the specified column but over all values of a
group. Thus the aggregate operator is evaluated individually for every
group. Thus the aggregate operator is evaluated separately for every
group.
</para>
@ -1547,7 +1527,7 @@ SELECT *
In our example the result will be empty because every supplier sells
at least one part. Note that we use S.SNO from the outer SELECT within
the WHERE clause of the inner SELECT. Here the subquery must be
evaluated for every tuple from the outer query, i.e. the value for
evaluated afresh for each tuple from the outer query, i.e. the value for
S.SNO is always taken from the current tuple of the outer SELECT.
</para>
</example>
@ -1605,7 +1585,7 @@ SELECT MAX(subtable.avgprice)
SELECT S.SNO, S.SNAME, S.CITY
FROM SUPPLIER S
WHERE S.SNAME = 'Jones'
UNION
UNION
SELECT S.SNO, S.SNAME, S.CITY
FROM SUPPLIER S
WHERE S.SNAME = 'Adams';
@ -1628,7 +1608,7 @@ gives the result:
SELECT S.SNO, S.SNAME, S.CITY
FROM SUPPLIER S
WHERE S.SNO > 1
INTERSECT
INTERSECT
SELECT S.SNO, S.SNAME, S.CITY
FROM SUPPLIER S
WHERE S.SNO < 3;
@ -1652,7 +1632,7 @@ SELECT S.SNO, S.SNAME, S.CITY
SELECT S.SNO, S.SNAME, S.CITY
FROM SUPPLIER S
WHERE S.SNO > 1
EXCEPT
EXCEPT
SELECT S.SNO, S.SNAME, S.CITY
FROM SUPPLIER S
WHERE S.SNO > 3;