<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Conditional Expressions</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 8.3.8 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Functions and Operators"
HREF="functions.html"><LINK
REL="PREVIOUS"
TITLE="Sequence Manipulation Functions"
HREF="functions-sequence.html"><LINK
REL="NEXT"
TITLE="Array Functions and Operators"
HREF="functions-array.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2009-09-04T05:13:45"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="5"
ALIGN="center"
VALIGN="bottom"
>PostgreSQL 8.3.8 Documentation</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="functions-sequence.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="functions.html"
>Fast Backward</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 9. Functions and Operators</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="top"
><A
HREF="functions.html"
>Fast Forward</A
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="top"
><A
HREF="functions-array.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-CONDITIONAL"
>9.16. Conditional Expressions</A
></H1
><A
NAME="AEN14425"
></A
><A
NAME="AEN14427"
></A
><P
> This section describes the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>-compliant conditional expressions
available in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> If your needs go beyond the capabilities of these conditional
expressions you might want to consider writing a stored procedure
in a more expressive programming language.
</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN14434"
>9.16.1. <TT
CLASS="LITERAL"
>CASE</TT
></A
></H2
><P
> The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> <TT
CLASS="TOKEN"
>CASE</TT
> expression is a
generic conditional expression, similar to if/else statements in
other languages:
</P><PRE
CLASS="SYNOPSIS"
>CASE WHEN <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> THEN <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
>
[<SPAN
CLASS="OPTIONAL"
>WHEN ...</SPAN
>]
[<SPAN
CLASS="OPTIONAL"
>ELSE <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
></SPAN
>]
END</PRE
><P>
<TT
CLASS="TOKEN"
>CASE</TT
> clauses can be used wherever
an expression is valid. <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> is an
expression that returns a <TT
CLASS="TYPE"
>boolean</TT
> result. If the result is true
then the value of the <TT
CLASS="TOKEN"
>CASE</TT
> expression is the
<TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
> that follows the condition. If the result is false any
subsequent <TT
CLASS="TOKEN"
>WHEN</TT
> clauses are searched in the same
manner. If no <TT
CLASS="TOKEN"
>WHEN</TT
>
<TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> is true then the value of the
case expression is the <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
> in the
<TT
CLASS="TOKEN"
>ELSE</TT
> clause. If the <TT
CLASS="TOKEN"
>ELSE</TT
> clause is
omitted and no condition matches, the result is null.
</P
><P
> An example:
</P><PRE
CLASS="SCREEN"
>SELECT * FROM test;
a
---
1
2
3
SELECT a,
CASE WHEN a=1 THEN 'one'
WHEN a=2 THEN 'two'
ELSE 'other'
END
FROM test;
a | case
---+-------
1 | one
2 | two
3 | other</PRE
><P>
</P
><P
> The data types of all the <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
>
expressions must be convertible to a single output type.
See <A
HREF="typeconv-union-case.html"
>Section 10.5</A
> for more detail.
</P
><P
> The following <SPAN
CLASS="QUOTE"
>"simple"</SPAN
> <TT
CLASS="TOKEN"
>CASE</TT
> expression is a
specialized variant of the general form above:
</P><PRE
CLASS="SYNOPSIS"
>CASE <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>
WHEN <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> THEN <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
>
[<SPAN
CLASS="OPTIONAL"
>WHEN ...</SPAN
>]
[<SPAN
CLASS="OPTIONAL"
>ELSE <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
></SPAN
>]
END</PRE
><P>
The
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> is computed and compared to
all the <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> specifications in the
<TT
CLASS="TOKEN"
>WHEN</TT
> clauses until one is found that is equal. If
no match is found, the <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
> in the
<TT
CLASS="TOKEN"
>ELSE</TT
> clause (or a null value) is returned. This is similar
to the <CODE
CLASS="FUNCTION"
>switch</CODE
> statement in C.
</P
><P
> The example above can be written using the simple
<TT
CLASS="TOKEN"
>CASE</TT
> syntax:
</P><PRE
CLASS="SCREEN"
>SELECT a,
CASE a WHEN 1 THEN 'one'
WHEN 2 THEN 'two'
ELSE 'other'
END
FROM test;
a | case
---+-------
1 | one
2 | two
3 | other</PRE
><P>
</P
><P
> A <TT
CLASS="TOKEN"
>CASE</TT
> expression does not evaluate any subexpressions
that are not needed to determine the result. For example, this is a
possible way of avoiding a division-by-zero failure:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN14484"
>9.16.2. <TT
CLASS="LITERAL"
>COALESCE</TT
></A
></H2
><A
NAME="AEN14487"
></A
><A
NAME="AEN14489"
></A
><A
NAME="AEN14491"
></A
><PRE
CLASS="SYNOPSIS"
><CODE
CLASS="FUNCTION"
>COALESCE</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ...</SPAN
>])</PRE
><P
> The <CODE
CLASS="FUNCTION"
>COALESCE</CODE
> function returns the first of its
arguments that is not null. Null is returned only if all arguments
are null. It is often used to substitute a default value for
null values when data is retrieved for display, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT COALESCE(description, short_description, '(none)') ...</PRE
><P>
</P
><P
> Like a <TT
CLASS="TOKEN"
>CASE</TT
> expression, <CODE
CLASS="FUNCTION"
>COALESCE</CODE
> will
not evaluate arguments that are not needed to determine the result;
that is, arguments to the right of the first non-null argument are
not evaluated. This SQL-standard function provides capabilities similar
to <CODE
CLASS="FUNCTION"
>NVL</CODE
> and <CODE
CLASS="FUNCTION"
>IFNULL</CODE
>, which are used in some other
database systems.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN14505"
>9.16.3. <TT
CLASS="LITERAL"
>NULLIF</TT
></A
></H2
><A
NAME="AEN14508"
></A
><PRE
CLASS="SYNOPSIS"
><CODE
CLASS="FUNCTION"
>NULLIF</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>value1</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>value2</I
></TT
>)</PRE
><P
> The <CODE
CLASS="FUNCTION"
>NULLIF</CODE
> function returns a null value if
<TT
CLASS="REPLACEABLE"
><I
>value1</I
></TT
> and <TT
CLASS="REPLACEABLE"
><I
>value2</I
></TT
>
are equal; otherwise it returns <TT
CLASS="REPLACEABLE"
><I
>value1</I
></TT
>.
This can be used to perform the inverse operation of the
<CODE
CLASS="FUNCTION"
>COALESCE</CODE
> example given above:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT NULLIF(value, '(none)') ...</PRE
><P>
</P
><P
> If <TT
CLASS="REPLACEABLE"
><I
>value1</I
></TT
> is <TT
CLASS="LITERAL"
>(none)</TT
>, return a null,
otherwise return <TT
CLASS="REPLACEABLE"
><I
>value1</I
></TT
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN14525"
>9.16.4. <TT
CLASS="LITERAL"
>GREATEST</TT
> and <TT
CLASS="LITERAL"
>LEAST</TT
></A
></H2
><A
NAME="AEN14529"
></A
><A
NAME="AEN14531"
></A
><PRE
CLASS="SYNOPSIS"
><CODE
CLASS="FUNCTION"
>GREATEST</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ...</SPAN
>])</PRE
><PRE
CLASS="SYNOPSIS"
><CODE
CLASS="FUNCTION"
>LEAST</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ...</SPAN
>])</PRE
><P
> The <CODE
CLASS="FUNCTION"
>GREATEST</CODE
> and <CODE
CLASS="FUNCTION"
>LEAST</CODE
> functions select the
largest or smallest value from a list of any number of expressions.
The expressions must all be convertible to a common data type, which
will be the type of the result
(see <A
HREF="typeconv-union-case.html"
>Section 10.5</A
> for details). NULL values
in the list are ignored. The result will be NULL only if all the
expressions evaluate to NULL.
</P
><P
> Note that <CODE
CLASS="FUNCTION"
>GREATEST</CODE
> and <CODE
CLASS="FUNCTION"
>LEAST</CODE
> are not in
the SQL standard, but are a common extension. Some other databases
make them return NULL if any argument is NULL, rather than only when
all are NULL.
</P
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="functions-sequence.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="functions-array.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Sequence Manipulation Functions</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="functions.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Array Functions and Operators</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
distrib
>
Mandriva
>
2010.0
>
i586
>
media
>
contrib-release
>
by-pkgid
>
415a9558429d583fc9b2617edd1d24a6
>
files
>
213
