database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-12-22 17:42:17 +03:00
Code Activity

Rip out table expression section from SQL syntax chapter and develop it

Browse Source 
into new chapter on query (SELECT) syntax.  In the end this should become
a narrative and example-filled counterpart to the SELECT reference page.
This commit is contained in:
Peter Eisentraut
2001-01-22 23:34:33 +00:00
parent e9c936ff38
commit 21a3857f1f
4 changed files with 832 additions and 532 deletions
Download Patch File Download Diff File Expand all files Collapse all files

538
doc/src/sgml/syntax.sgml
View File

@@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.36 2001/01/21 22:02:01 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.37 2001/01/22 23:34:33 petere Exp $
-->
<chapter id="sql-syntax">
@@ -743,7 +743,7 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
</para>
<para>
In addition to this list, there are a number of contructs that can
In addition to this list, there are a number of constructs 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
@@ -763,15 +763,15 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
<para>
A column can be referenced in the form:
<synopsis>
<replaceable>corelation</replaceable>.<replaceable>columnname</replaceable> `['<replaceable>subscript</replaceable>`]'
<replaceable>correlation</replaceable>.<replaceable>columnname</replaceable> `['<replaceable>subscript</replaceable>`]'
</synopsis>
<replaceable>corelation</replaceable> is either the name of a
<replaceable>correlation</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
while other correlation names can be used in any SQL statement.)
The correlation 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
@@ -895,8 +895,8 @@ sqrt(2)
The precedence and associativity of the operators is hard-wired
into the parser. Most operators have the same precedence and are
left-associative. This may lead to non-intuitive behavior; for
example the boolean operators "&lt;" and "&gt;" have a different
precedence than the boolean operators "&lt;=" and "&gt;=". Also,
example the Boolean operators "&lt;" and "&gt;" have a different
precedence than the Boolean operators "&lt;=" and "&gt;=". Also,
you will sometimes need to add parentheses when using combinations
of binary and unary operators. For instance
<programlisting>
@@ -917,7 +917,7 @@ SELECT (5 &amp;) ~ 6;
<tgroup cols="2">
<thead>
<row>
<entry>OperatorElement</entry>
<entry>Operator/Element</entry>
<entry>Associativity</entry>
<entry>Description</entry>
</row>
@@ -1057,526 +1057,6 @@ SELECT (5 &amp;) ~ 6;
</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 &gt; 5
</programlisting>
and
<programlisting>
FROM a INNER JOIN b ON (a.id = b.id) WHERE b.val &gt; 5
</programlisting>
or perhaps even
<programlisting>
FROM a NATURAL JOIN b WHERE b.val &gt; 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>
<!-- Keep this comment at the end of the file