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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 V1.79.1" /><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 12.4 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="note"><h3 class="title">Note</h3><p>
     Although <code class="token">COALESCE</code>, <code class="token">GREATEST</code>, and
     <code class="token">LEAST</code> are syntactically similar to functions, they are
     not ordinary functions, and thus cannot be used with explicit
     <code class="token">VARIADIC</code> array arguments.
    </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 &lt;&gt; 0 THEN y/x &gt; 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-1.5.8.22.8.2" class="indexterm"></a><a id="id-1.5.8.22.8.3" class="indexterm"></a><a id="id-1.5.8.22.8.4" 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-1.5.8.22.9.2" 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-1.5.8.22.10.2" class="indexterm"></a><a id="id-1.5.8.22.10.3" 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.
   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="functions-sequence.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="functions.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="functions-array.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">9.16. Sequence Manipulation Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 9.18. Array Functions and Operators</td></tr></table></div></body></html>