1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
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 15.7 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 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 9.19. Array Functions and Operators</td></tr></table></div></body></html>
|
