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><A
NAME="QUERIES-OVERVIEW"
>7.1. Overview</A
></H1
><P
> The process of retrieving or the command to retrieve data from a
database is called a <I
CLASS="FIRSTTERM"
>query</I
>. In SQL the
<A
HREF="sql-select.html"
>SELECT</A
> command is
used to specify queries. The general syntax of the
<TT
CLASS="COMMAND"
>SELECT</TT
> command is
</P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
>WITH <TT
CLASS="REPLACEABLE"
><I
>with_queries</I
></TT
></SPAN
>] SELECT <TT
CLASS="REPLACEABLE"
><I
>select_list</I
></TT
> FROM <TT
CLASS="REPLACEABLE"
><I
>table_expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
><TT
CLASS="REPLACEABLE"
><I
>sort_specification</I
></TT
></SPAN
>]</PRE
><P>
The following sections describe the details of the select list, the
table expression, and the sort specification. <TT
CLASS="LITERAL"
>WITH</TT
>
queries are treated last since they are an advanced feature.
</P
><P
> A simple kind of query has the form:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM table1;</PRE
><P>
Assuming that there is a table called <TT
CLASS="LITERAL"
>table1</TT
>,
this command would retrieve all rows and all user-defined columns from
<TT
CLASS="LITERAL"
>table1</TT
>. (The method of retrieval depends on the
client application. For example, the
<SPAN
CLASS="APPLICATION"
>psql</SPAN
> program will display an ASCII-art
table on the screen, while client libraries will offer functions to
extract individual values from the query result.) The select list
specification <TT
CLASS="LITERAL"
>*</TT
> means all columns that the table
expression happens to provide. A select list can also select a
subset of the available columns or make calculations using the
columns. For example, if
<TT
CLASS="LITERAL"
>table1</TT
> has columns named <TT
CLASS="LITERAL"
>a</TT
>,
<TT
CLASS="LITERAL"
>b</TT
>, and <TT
CLASS="LITERAL"
>c</TT
> (and perhaps others) you can make
the following query:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a, b + c FROM table1;</PRE
><P>
(assuming that <TT
CLASS="LITERAL"
>b</TT
> and <TT
CLASS="LITERAL"
>c</TT
> are of a numerical
data type).
See <A
HREF="queries-select-lists.html"
>Section 7.3</A
> for more details.
</P
><P
> <TT
CLASS="LITERAL"
>FROM table1</TT
> is a simple kind of
table expression: it reads just one table. In general, table
expressions can be complex constructs of base tables, joins, and
subqueries. But you can also omit the table expression entirely and
use the <TT
CLASS="COMMAND"
>SELECT</TT
> command as a calculator:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT 3 * 4;</PRE
><P>
This is more useful if the expressions in the select list return
varying results. For example, you could call a function this way:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT random();</PRE
><P>
</P
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