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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-CONDITIONAL"
>9.12. Conditional Expressions</A
></H1
><A
NAME="AEN11372"
></A
><A
NAME="AEN11374"
></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="AEN11381"
>9.12.1. <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 <VAR
CLASS="REPLACEABLE"
>condition</VAR
> THEN <VAR
CLASS="REPLACEABLE"
>result</VAR
>
[<SPAN
CLASS="OPTIONAL"
>WHEN ...</SPAN
>]
[<SPAN
CLASS="OPTIONAL"
>ELSE <VAR
CLASS="REPLACEABLE"
>result</VAR
></SPAN
>]
END</PRE
><P>
<TT
CLASS="TOKEN"
>CASE</TT
> clauses can be used wherever
an expression is valid. <VAR
CLASS="REPLACEABLE"
>condition</VAR
> 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
<VAR
CLASS="REPLACEABLE"
>result</VAR
> 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
>
<VAR
CLASS="REPLACEABLE"
>condition</VAR
> is true then the value of the
case expression is the <VAR
CLASS="REPLACEABLE"
>result</VAR
> 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 <VAR
CLASS="REPLACEABLE"
>result</VAR
>
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 <VAR
CLASS="REPLACEABLE"
>expression</VAR
>
WHEN <VAR
CLASS="REPLACEABLE"
>value</VAR
> THEN <VAR
CLASS="REPLACEABLE"
>result</VAR
>
[<SPAN
CLASS="OPTIONAL"
>WHEN ...</SPAN
>]
[<SPAN
CLASS="OPTIONAL"
>ELSE <VAR
CLASS="REPLACEABLE"
>result</VAR
></SPAN
>]
END</PRE
><P>
The
<VAR
CLASS="REPLACEABLE"
>expression</VAR
> is computed and compared to
all the <VAR
CLASS="REPLACEABLE"
>value</VAR
> specifications in the
<TT
CLASS="TOKEN"
>WHEN</TT
> clauses until one is found that is equal. If
no match is found, the <VAR
CLASS="REPLACEABLE"
>result</VAR
> 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="AEN11431"
>9.12.2. <TT
CLASS="LITERAL"
>COALESCE</TT
></A
></H2
><A
NAME="AEN11434"
></A
><PRE
CLASS="SYNOPSIS"
><CODE
CLASS="FUNCTION"
>COALESCE</CODE
>(<VAR
CLASS="REPLACEABLE"
>value</VAR
> [<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. This is often useful 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.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN11446"
>9.12.3. <TT
CLASS="LITERAL"
>NULLIF</TT
></A
></H2
><A
NAME="AEN11449"
></A
><PRE
CLASS="SYNOPSIS"
><CODE
CLASS="FUNCTION"
>NULLIF</CODE
>(<VAR
CLASS="REPLACEABLE"
>value1</VAR
>, <VAR
CLASS="REPLACEABLE"
>value2</VAR
>)</PRE
><P
> The <CODE
CLASS="FUNCTION"
>NULLIF</CODE
> function returns a null value if and only
if <VAR
CLASS="REPLACEABLE"
>value1</VAR
> and
<VAR
CLASS="REPLACEABLE"
>value2</VAR
> are equal. Otherwise it returns
<VAR
CLASS="REPLACEABLE"
>value1</VAR
>. 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>
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