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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>9.17. Conditional Expressions</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="functions-sequence.html" title="9.16. Sequence Manipulation Functions" /><link rel="next" href="functions-array.html" title="9.18. Array Functions and Operators" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">9.17. Conditional Expressions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="functions-sequence.html" title="9.16. Sequence Manipulation Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="functions.html" title="Chapter 9. Functions and Operators">Up</a></td><th width="60%" align="center">Chapter 9. Functions and Operators</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="functions-array.html" title="9.18. Array Functions and Operators">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="FUNCTIONS-CONDITIONAL"><div class="titlepage"><div><div><h2 class="title" style="clear: both">9.17. Conditional Expressions</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="functions-conditional.html#FUNCTIONS-CASE">9.17.1. <code class="literal">CASE</code></a></span></dt><dt><span class="sect2"><a href="functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL">9.17.2. <code class="literal">COALESCE</code></a></span></dt><dt><span class="sect2"><a href="functions-conditional.html#FUNCTIONS-NULLIF">9.17.3. <code class="literal">NULLIF</code></a></span></dt><dt><span class="sect2"><a href="functions-conditional.html#FUNCTIONS-GREATEST-LEAST">9.17.4. <code class="literal">GREATEST</code> and <code class="literal">LEAST</code></a></span></dt></dl></div><a id="id-1.5.8.22.2" class="indexterm"></a><a id="id-1.5.8.22.3" class="indexterm"></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"><h3 class="title">Tip</h3><p>
If your needs go beyond the capabilities of these conditional
expressions, you might want to consider writing a server-side function
in a more expressive programming language.
</p></div><div class="sect2" id="FUNCTIONS-CASE"><div class="titlepage"><div><div><h3 class="title">9.17.1. <code class="literal">CASE</code></h3></div></div></div><p>
The <acronym class="acronym">SQL</acronym> <code class="token">CASE</code> expression is a
generic conditional expression, similar to if/else statements in
other programming languages:
</p><pre class="synopsis">
CASE WHEN <em class="replaceable"><code>condition</code></em> THEN <em class="replaceable"><code>result</code></em>
[<span class="optional">WHEN ...</span>]
[<span class="optional">ELSE <em class="replaceable"><code>result</code></em></span>]
END
</pre><p>
<code class="token">CASE</code> clauses can be used wherever
an expression is valid. Each <em class="replaceable"><code>condition</code></em> is an
expression that returns a <code class="type">boolean</code> result. If the condition's
result is true, the value of the <code class="token">CASE</code> expression is the
<em class="replaceable"><code>result</code></em> that follows the condition, and the
remainder of the <code class="token">CASE</code> expression is not processed. If the
condition's result is not true, any subsequent <code class="token">WHEN</code> clauses
are examined in the same manner. If no <code class="token">WHEN</code>
<em class="replaceable"><code>condition</code></em> yields true, the value of the
<code class="token">CASE</code> expression is the <em class="replaceable"><code>result</code></em> of the
<code class="token">ELSE</code> clause. If the <code class="token">ELSE</code> 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 <em class="replaceable"><code>result</code></em>
expressions must be convertible to a single output type.
See <a class="xref" href="typeconv-union-case.html" title="10.5. UNION, CASE, and Related Constructs">Section 10.5</a> for more details.
</p><p>
There is a <span class="quote">“<span class="quote">simple</span>”</span> form of <code class="token">CASE</code> expression
that is a variant of the general form above:
</p><pre class="synopsis">
CASE <em class="replaceable"><code>expression</code></em>
WHEN <em class="replaceable"><code>value</code></em> THEN <em class="replaceable"><code>result</code></em>
[<span class="optional">WHEN ...</span>]
[<span class="optional">ELSE <em class="replaceable"><code>result</code></em></span>]
END
</pre><p>
The first
<em class="replaceable"><code>expression</code></em> is computed, then compared to
each of the <em class="replaceable"><code>value</code></em> expressions in the
<code class="token">WHEN</code> clauses until one is found that is equal to it. If
no match is found, the <em class="replaceable"><code>result</code></em> of the
<code class="token">ELSE</code> 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
<code class="token">CASE</code> 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 <code class="token">CASE</code> 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"><h3 class="title">Note</h3><p>
As described in <a class="xref" href="sql-expressions.html#SYNTAX-EXPRESS-EVAL" title="4.2.14. Expression Evaluation Rules">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">“<span class="quote"><code class="token">CASE</code>
evaluates only necessary subexpressions</span>”</span> is not ironclad. For
example a constant <code class="literal">1/0</code> subexpression will usually result in
a division-by-zero failure at planning time, even if it's within
a <code class="token">CASE</code> arm that would never be entered at run time.
</p></div></div><div class="sect2" id="FUNCTIONS-COALESCE-NVL-IFNULL"><div class="titlepage"><div><div><h3 class="title">9.17.2. <code class="literal">COALESCE</code></h3></div></div></div><a id="id-1.5.8.22.7.2" class="indexterm"></a><a id="id-1.5.8.22.7.3" class="indexterm"></a><a id="id-1.5.8.22.7.4" class="indexterm"></a><pre class="synopsis">
<code class="function">COALESCE</code>(<em class="replaceable"><code>value</code></em> [<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 <code class="varname">description</code> if it is not null, otherwise
<code class="varname">short_description</code> if it is not null, otherwise <code class="literal">(none)</code>.
</p><p>
Like a <code class="token">CASE</code> 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" id="FUNCTIONS-NULLIF"><div class="titlepage"><div><div><h3 class="title">9.17.3. <code class="literal">NULLIF</code></h3></div></div></div><a id="id-1.5.8.22.8.2" class="indexterm"></a><pre class="synopsis">
<code class="function">NULLIF</code>(<em class="replaceable"><code>value1</code></em>, <em class="replaceable"><code>value2</code></em>)
</pre><p>
The <code class="function">NULLIF</code> function returns a null value if
<em class="replaceable"><code>value1</code></em> equals <em class="replaceable"><code>value2</code></em>;
otherwise it returns <em class="replaceable"><code>value1</code></em>.
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 <code class="literal">value</code> is <code class="literal">(none)</code>,
null is returned, otherwise the value of <code class="literal">value</code>
is returned.
</p></div><div class="sect2" id="FUNCTIONS-GREATEST-LEAST"><div class="titlepage"><div><div><h3 class="title">9.17.4. <code class="literal">GREATEST</code> and <code class="literal">LEAST</code></h3></div></div></div><a id="id-1.5.8.22.9.2" class="indexterm"></a><a id="id-1.5.8.22.9.3" class="indexterm"></a><pre class="synopsis">
<code class="function">GREATEST</code>(<em class="replaceable"><code>value</code></em> [<span class="optional">, ...</span>])
</pre><pre class="synopsis">
<code class="function">LEAST</code>(<em class="replaceable"><code>value</code></em> [<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 class="xref" href="typeconv-union-case.html" title="10.5. UNION, CASE, and Related Constructs">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.
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