diff options
Diffstat (limited to 'doc/src/sgml/html/functions-conditional.html')
| -rw-r--r-- | doc/src/sgml/html/functions-conditional.html | 187 |
1 files changed, 187 insertions, 0 deletions
diff --git a/doc/src/sgml/html/functions-conditional.html b/doc/src/sgml/html/functions-conditional.html new file mode 100644 index 0000000..0ce4001 --- /dev/null +++ b/doc/src/sgml/html/functions-conditional.html @@ -0,0 +1,187 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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>9.18. 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="9.17. Sequence Manipulation Functions" /><link rel="next" href="functions-array.html" title="9.19. Array Functions and Operators" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">9.18. Conditional Expressions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="functions-sequence.html" title="9.17. 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 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="functions-array.html" title="9.19. Array Functions and Operators">Next</a></td></tr></table><hr /></div><div class="sect1" id="FUNCTIONS-CONDITIONAL"><div class="titlepage"><div><div><h2 class="title" style="clear: both">9.18. Conditional Expressions</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="functions-conditional.html#FUNCTIONS-CASE">9.18.1. <code class="literal">CASE</code></a></span></dt><dt><span class="sect2"><a href="functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL">9.18.2. <code class="literal">COALESCE</code></a></span></dt><dt><span class="sect2"><a href="functions-conditional.html#FUNCTIONS-NULLIF">9.18.3. <code class="literal">NULLIF</code></a></span></dt><dt><span class="sect2"><a href="functions-conditional.html#FUNCTIONS-GREATEST-LEAST">9.18.4. <code class="literal">GREATEST</code> and <code class="literal">LEAST</code></a></span></dt></dl></div><a id="id-1.5.8.24.2" class="indexterm"></a><a id="id-1.5.8.24.3" 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">9.18.1. <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">9.18.2. <code class="literal">COALESCE</code></h3></div></div></div><a id="id-1.5.8.24.8.2" class="indexterm"></a><a id="id-1.5.8.24.8.3" class="indexterm"></a><a id="id-1.5.8.24.8.4" 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> + The arguments 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). + </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">9.18.3. <code class="literal">NULLIF</code></h3></div></div></div><a id="id-1.5.8.24.9.2" 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> + 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><p> + The two arguments must be of comparable types. + To be specific, they are compared exactly as if you had + written <code class="literal"><em class="replaceable"><code>value1</code></em> + = <em class="replaceable"><code>value2</code></em></code>, so there must be a + suitable <code class="literal">=</code> operator available. + </p><p> + The result has the same type as the first argument — but there is + a subtlety. What is actually returned is the first argument of the + implied <code class="literal">=</code> operator, and in some cases that will have + been promoted to match the second argument's type. For + example, <code class="literal">NULLIF(1, 2.2)</code> yields <code class="type">numeric</code>, + because there is no <code class="type">integer</code> <code class="literal">=</code> + <code class="type">numeric</code> operator, + only <code class="type">numeric</code> <code class="literal">=</code> <code class="type">numeric</code>. + </p></div><div class="sect2" id="FUNCTIONS-GREATEST-LEAST"><div class="titlepage"><div><div><h3 class="title">9.18.4. <code class="literal">GREATEST</code> and <code class="literal">LEAST</code></h3></div></div></div><a id="id-1.5.8.24.10.2" class="indexterm"></a><a id="id-1.5.8.24.10.3" 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" title="9.17. Sequence Manipulation Functions">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="functions.html" title="Chapter 9. Functions and Operators">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="functions-array.html" title="9.19. Array Functions and Operators">Next</a></td></tr><tr><td width="40%" align="left" valign="top">9.17. Sequence Manipulation Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 9.19. Array Functions and Operators</td></tr></table></div></body></html>
\ No newline at end of file |