Still needs to be filled with more information, but it gives us a
framework to have a User's Guide with complete coverage of the basic
SQL operations. Move arrays into data type chapter, inheritance into
DDL chapter (for now).
Make <comment>s show up in the output while the version number ends in
"devel".
Allow cross-book references with entities &cite-user; etc.
The previous chapters explained how to create tables, how to fill
them with data, and how to manipulate that data. Now we finally
discuss how to retrieve the data out of the database.
</para>
<sect1 id="queries-overview">
<title>Overview</title>
<para>
A <firstterm>query</firstterm> is the process of retrieving or the command
to retrieve data from a database. In SQL the <command>SELECT</command>
command is used to specify queries. The general syntax of the
<command>SELECT</command> command is
The process of retrieving or the command to retrieve data from a
database is called a <firstterm>query</firstterm>. In SQL the
<command>SELECT</command> command is used to specify queries. The
general syntax of the <command>SELECT</command> command is
<synopsis>
SELECT <replaceable>select_list</replaceable> FROM <replaceable>table_expression</replaceable> <optional><replaceable>sort_specification</replaceable></optional>
</synopsis>
@ -114,14 +120,14 @@ 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, and CROSS JOIN are supported.
type. Inner, outer, and cross-joins are available.
</para>
<variablelist>
<title>Join Types</title>
<varlistentry>
<term>CROSS JOIN</term>
<term>Cross-join</term>
<indexterm>
<primary>joins</primary>
@ -244,7 +250,7 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
<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, a joined row is returned with null values in columns of
T2. Thus, the joined table unconditionally has at least one
row for each row in T1.
</para>
@ -258,7 +264,7 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
<para>
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, 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>
@ -460,7 +466,7 @@ WHERE <replaceable>search_condition</replaceable>
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
table, otherwise (that is, if the result is false or null) it is
discarded. 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.
Peter Eisentraut
23 years ago