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><H1
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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 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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