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Improve documentation of JOIN syntax. Explain NATURAL as an alternative 

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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108
doc/src/sgml/queries.sgml
View File

@ -1,4 +1,4 @@
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/queries.sgml,v 1.4 2001/02/13 21:13:11 petere Exp $ -->
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/queries.sgml,v 1.5 2001/02/15 04:10:54 tgl Exp $ -->
<chapter id="queries">
<title>Queries</title>
@ -105,7 +105,7 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
<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.
type. INNER, OUTER, and CROSS JOIN are supported.
</para>
<variablelist>
@ -122,10 +122,10 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
<para>
For each combination of rows from
<replaceable>T1</replaceable> and
<replaceable>T2</replaceable> the derived table will contain a
<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
the tables have N and M rows respectively, the joined
table will have N * M rows. A cross join is equivalent to an
<literal>INNER JOIN ON TRUE</literal>.
</para>
@ -148,32 +148,55 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
<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> )
<replaceable>T1</replaceable> NATURAL { <optional>INNER</optional> | { LEFT | RIGHT | FULL } <optional>OUTER</optional> } JOIN <replaceable>T2</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.
<token>FULL</token> imply an OUTER JOIN.
</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
ON or USING clause, or implicitly by the word NATURAL. The join
condition determines which rows from the two source tables are
considered to <quote>match</quote>, as explained in detail below.
</para>
<para>
The ON clause is the most general kind of join condition: it takes a
Boolean value expression of the same kind as is used in a WHERE
clause. A pair of rows from T1 and T2 match if the ON expression
evaluates to TRUE for them.
</para>
<para>
USING is a shorthand notation: it 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
must have in common, and forms a join condition specifying equality
of each of these pairs of columns. Furthermore, the output of
a JOIN USING has one column for each of the equated pairs of
input columns, followed by all of the other 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 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>
<para>
Finally, NATURAL is a shorthand form of USING: it forms a USING
list consisting of exactly those column names that appear in both
input tables. As with USING, these columns appear only once in
the output table.
</para>
<para>
The possible types of qualified JOIN are:
</para>
<variablelist>
<varlistentry>
<term>INNER JOIN</term>
@ -205,7 +228,10 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
<listitem>
<para>
This is the converse of a left join: the result table will
First, an INNER JOIN is performed. Then, for each row in T2
that does not satisfy the join condition with any row in
T1, a joined row is returned with NULL values in columns of
T1. This is the converse of a left join: the result table will
unconditionally have a row for each row in T2.
</para>
</listitem>
@ -228,22 +254,6 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
</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
result is the same as a qualified join with a USING clause that lists
all the common column names of the two tables.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
@ -270,8 +280,9 @@ FROM (SELECT * FROM table1) AS alias_name
<para>
This example is equivalent to <literal>FROM table1 AS
alias_name</literal>. Many subqueries can be written as table
joins instead.
alias_name</literal>. More interesting cases, which can't be
reduced to a plain join, arise when the subquery involves grouping
or aggregation.
</para>
</sect3>
@ -331,13 +342,28 @@ FROM <replaceable>table_reference</replaceable> <replaceable>alias</replaceable>
<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
In this form,
in addition to renaming the table as described above, the columns
of the table are also given temporary names for use by the surrounding
query. If fewer column
aliases are specified than the actual table has columns, the remaining
columns are not renamed. This syntax is especially useful for
self-joins or subqueries.
</para>
<para>
When an alias is applied to the output of a JOIN clause, using any of
these forms, the alias hides the original names within the JOIN.
For example,
<programlisting>
SELECT a.* FROM my_table AS a JOIN your_table AS b ON ...
</programlisting>
is valid SQL, but
<programlisting>
SELECT a.* FROM (my_table AS a JOIN your_table AS b ON ...) AS c
</programlisting>
is not valid: the table alias A is not visible outside the alias C.
</para>
</sect3>
<sect3 id="queries-table-expression-examples">
@ -442,13 +468,13 @@ FROM FDT WHERE
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
subqueries as value expressions. 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 also 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.
column name adds clarity even when it is not needed. This shows how
the column naming scope of an outer query extends into its inner queries.
</para>
</sect2>
@ -488,8 +514,8 @@ SELECT <replaceable>select_list</replaceable> FROM ... <optional>WHERE ...</opti
<function>sum(sales)</function> on a table grouped by product code
gives the total sales for each product, not the total sales on all
products. Aggregates computed on the ungrouped columns are
representative of the group, whereas their individual values may
not be.
representative of the group, whereas individual values of an ungrouped
column are not.
</para>
<para>
@ -583,7 +609,7 @@ SELECT tbl1.a, tbl2.b, tbl1.c FROM ...
<para>
If an arbitrary value expression is used in the select list, it
conceptually adds a new virtual column to the returned table. The
value expression is effectively evaluated once for each retrieved
value expression is evaluated once for each retrieved
row, with the row's values substituted for any column references. But
the expressions in the select list do not have to reference any
columns in the table expression of the FROM clause; they could be

162
doc/src/sgml/sql.sgml
View File

@ -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;