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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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>Conditional Expressions</TITLE
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-CONDITIONAL"
>9.17. Conditional Expressions</A
></H1
><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="FUNCTIONS-CASE"
>9.17.1. <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 programming 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. Each <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> is an
expression that returns a <TT
CLASS="TYPE"
>boolean</TT
> result. If the condition's
result is true, the value of the <TT
CLASS="TOKEN"
>CASE</TT
> expression is the
<TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
> that follows the condition, and the
remainder of the <TT
CLASS="TOKEN"
>CASE</TT
> expression is not processed. If the
condition's result is not true, any subsequent <TT
CLASS="TOKEN"
>WHEN</TT
> clauses
are examined in the same manner. If no <TT
CLASS="TOKEN"
>WHEN</TT
>
<TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> yields true, the value of the
<TT
CLASS="TOKEN"
>CASE</TT
> expression is the <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
> of the
<TT
CLASS="TOKEN"
>ELSE</TT
> clause. If the <TT
CLASS="TOKEN"
>ELSE</TT
> clause is
omitted and no condition is true, 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 details.
</P
><P
> There is a <SPAN
CLASS="QUOTE"
>"simple"</SPAN
> form of <TT
CLASS="TOKEN"
>CASE</TT
> expression
that is a 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 first
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> is computed, then compared to
each of the <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> expressions in the
<TT
CLASS="TOKEN"
>WHEN</TT
> clauses until one is found that is equal to it. If
no match is found, the <TT
CLASS="REPLACEABLE"
><I
>result</I
></TT
> of 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
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> As described in <A
HREF="sql-expressions.html#SYNTAX-EXPRESS-EVAL"
>Section 4.2.14</A
>, there are various
situations in which subexpressions of an expression are evaluated at
different times, so that the principle that <SPAN
CLASS="QUOTE"
>"<TT
CLASS="TOKEN"
>CASE</TT
>
evaluates only necessary subexpressions"</SPAN
> is not ironclad. For
example a constant <TT
CLASS="LITERAL"
>1/0</TT
> subexpression will usually result in
a division-by-zero failure at planning time, even if it's within
a <TT
CLASS="TOKEN"
>CASE</TT
> arm that would never be entered at run time.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-COALESCE-NVL-IFNULL"
>9.17.2. <TT
CLASS="LITERAL"
>COALESCE</TT
></A
></H2
><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>
This returns <TT
CLASS="VARNAME"
>description</TT
> if it is not null, otherwise
<TT
CLASS="VARNAME"
>short_description</TT
> if it is not null, otherwise <TT
CLASS="LITERAL"
>(none)</TT
>.
</P
><P
> Like a <TT
CLASS="TOKEN"
>CASE</TT
> expression, <CODE
CLASS="FUNCTION"
>COALESCE</CODE
> only
evaluates the arguments that are 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="FUNCTIONS-NULLIF"
>9.17.3. <TT
CLASS="LITERAL"
>NULLIF</TT
></A
></H2
><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
> equals <TT
CLASS="REPLACEABLE"
><I
>value2</I
></TT
>;
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
> In this example, if <TT
CLASS="LITERAL"
>value</TT
> is <TT
CLASS="LITERAL"
>(none)</TT
>,
null is returned, otherwise the value of <TT
CLASS="LITERAL"
>value</TT
>
is returned.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-GREATEST-LEAST"
>9.17.4. <TT
CLASS="LITERAL"
>GREATEST</TT
> and <TT
CLASS="LITERAL"
>LEAST</TT
></A
></H2
><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
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