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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>6.3. Conditional Expressions — Presto 0.184 Documentation</title>
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<body role="document">
<div class="header">
<h1 class="heading"><a href="../index.html">
<span>Presto 0.184 Documentation</span></a></h1>
<h2 class="heading"><span>6.3. Conditional Expressions</span></h2>
</div>
<div class="topnav">
<p class="nav">
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« <a href="comparison.html">6.2. Comparison Functions and Operators</a>
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<div class="content">
<div class="section" id="conditional-expressions">
<h1>6.3. Conditional Expressions</h1>
<div class="section" id="case">
<h2>CASE</h2>
<p>The standard SQL <code class="docutils literal"><span class="pre">CASE</span></code> expression has two forms.
The “simple” form searches each <code class="docutils literal"><span class="pre">value</span></code> expression from left to right
until it finds one that equals <code class="docutils literal"><span class="pre">expression</span></code>:</p>
<div class="highlight-none"><div class="highlight"><pre><span></span>CASE expression
WHEN value THEN result
[ WHEN ... ]
[ ELSE result ]
END
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">result</span></code> for the matching <code class="docutils literal"><span class="pre">value</span></code> is returned.
If no match is found, the <code class="docutils literal"><span class="pre">result</span></code> from the <code class="docutils literal"><span class="pre">ELSE</span></code> clause is
returned if it exists, otherwise null is returned. Example:</p>
<div class="highlight-sql"><div class="highlight"><pre><span></span><span class="k">SELECT</span> <span class="n">a</span><span class="p">,</span>
<span class="k">CASE</span> <span class="n">a</span>
<span class="k">WHEN</span> <span class="mi">1</span> <span class="k">THEN</span> <span class="s1">'one'</span>
<span class="k">WHEN</span> <span class="mi">2</span> <span class="k">THEN</span> <span class="s1">'two'</span>
<span class="k">ELSE</span> <span class="s1">'many'</span>
<span class="k">END</span>
</pre></div>
</div>
<p>The “searched” form evaluates each boolean <code class="docutils literal"><span class="pre">condition</span></code> from left
to right until one is true and returns the matching <code class="docutils literal"><span class="pre">result</span></code>:</p>
<div class="highlight-none"><div class="highlight"><pre><span></span>CASE
WHEN condition THEN result
[ WHEN ... ]
[ ELSE result ]
END
</pre></div>
</div>
<p>If no conditions are true, the <code class="docutils literal"><span class="pre">result</span></code> from the <code class="docutils literal"><span class="pre">ELSE</span></code> clause is
returned if it exists, otherwise null is returned. Example:</p>
<div class="highlight-sql"><div class="highlight"><pre><span></span><span class="k">SELECT</span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">,</span>
<span class="k">CASE</span>
<span class="k">WHEN</span> <span class="n">a</span> <span class="o">=</span> <span class="mi">1</span> <span class="k">THEN</span> <span class="s1">'aaa'</span>
<span class="k">WHEN</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">2</span> <span class="k">THEN</span> <span class="s1">'bbb'</span>
<span class="k">ELSE</span> <span class="s1">'ccc'</span>
<span class="k">END</span>
</pre></div>
</div>
</div>
<div class="section" id="if">
<h2>IF</h2>
<p>The <code class="docutils literal"><span class="pre">IF</span></code> function is actually a language construct
that is equivalent to the following <code class="docutils literal"><span class="pre">CASE</span></code> expression:</p>
<blockquote>
<div><div class="highlight-none"><div class="highlight"><pre><span></span>CASE
WHEN condition THEN true_value
[ ELSE false_value ]
END
</pre></div>
</div>
</div></blockquote>
<dl class="function">
<dt>
<code class="descname">if</code><span class="sig-paren">(</span><em>condition</em>, <em>true_value</em><span class="sig-paren">)</span></dt>
<dd><p>Evaluates and returns <code class="docutils literal"><span class="pre">true_value</span></code> if <code class="docutils literal"><span class="pre">condition</span></code> is true,
otherwise null is returned and <code class="docutils literal"><span class="pre">true_value</span></code> is not evaluated.</p>
</dd></dl>
<dl class="function">
<dt>
<code class="descname">if</code><span class="sig-paren">(</span><em>condition</em>, <em>true_value</em>, <em>false_value</em><span class="sig-paren">)</span></dt>
<dd><p>Evaluates and returns <code class="docutils literal"><span class="pre">true_value</span></code> if <code class="docutils literal"><span class="pre">condition</span></code> is true,
otherwise evaluates and returns <code class="docutils literal"><span class="pre">false_value</span></code>.</p>
</dd></dl>
</div>
<div class="section" id="coalesce">
<h2>COALESCE</h2>
<dl class="function">
<dt>
<code class="descname">coalesce</code><span class="sig-paren">(</span><em>value1</em>, <em>value2</em><span class="optional">[</span>, <em>...</em><span class="optional">]</span><span class="sig-paren">)</span></dt>
<dd><p>Returns the first non-null <code class="docutils literal"><span class="pre">value</span></code> in the argument list.
Like a <code class="docutils literal"><span class="pre">CASE</span></code> expression, arguments are only evaluated if necessary.</p>
</dd></dl>
</div>
<div class="section" id="nullif">
<h2>NULLIF</h2>
<dl class="function">
<dt>
<code class="descname">nullif</code><span class="sig-paren">(</span><em>value1</em>, <em>value2</em><span class="sig-paren">)</span></dt>
<dd><p>Returns null if <code class="docutils literal"><span class="pre">value1</span></code> equals <code class="docutils literal"><span class="pre">value2</span></code>, otherwise returns <code class="docutils literal"><span class="pre">value1</span></code>.</p>
</dd></dl>
</div>
<div class="section" id="try">
<h2>TRY</h2>
<dl class="function">
<dt>
<code class="descname">try</code><span class="sig-paren">(</span><em>expression</em><span class="sig-paren">)</span></dt>
<dd><p>Evaluate an expression and handle certain types of errors by returning
<code class="docutils literal"><span class="pre">NULL</span></code>.</p>
</dd></dl>
<p>In cases where it is preferable that queries produce <code class="docutils literal"><span class="pre">NULL</span></code> or default values
instead of failing when corrupt or invalid data is encountered, the <code class="docutils literal"><span class="pre">TRY</span></code>
function may be useful. To specify default values, the <code class="docutils literal"><span class="pre">TRY</span></code> function can be
used in conjunction with the <code class="docutils literal"><span class="pre">COALESCE</span></code> function.</p>
<p>The following errors are handled by <code class="docutils literal"><span class="pre">TRY</span></code>:</p>
<ul class="simple">
<li>Division by zero</li>
<li>Invalid cast or function argument</li>
<li>Numeric value out of range</li>
</ul>
<div class="section" id="examples">
<h3>Examples</h3>
<p>Source table with some invalid data:</p>
<div class="highlight-sql"><div class="highlight"><pre><span></span><span class="k">SELECT</span> <span class="o">*</span> <span class="k">FROM</span> <span class="n">shipping</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-none"><div class="highlight"><pre><span></span> origin_state | origin_zip | packages | total_cost
--------------+------------+----------+------------
California | 94131 | 25 | 100
California | P332a | 5 | 72
California | 94025 | 0 | 155
New Jersey | 08544 | 225 | 490
(4 rows)
</pre></div>
</div>
<p>Query failure without <code class="docutils literal"><span class="pre">TRY</span></code>:</p>
<div class="highlight-sql"><div class="highlight"><pre><span></span><span class="k">SELECT</span> <span class="k">CAST</span><span class="p">(</span><span class="n">origin_zip</span> <span class="k">AS</span> <span class="nb">BIGINT</span><span class="p">)</span> <span class="k">FROM</span> <span class="n">shipping</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-none"><div class="highlight"><pre><span></span>Query failed: Can not cast 'P332a' to BIGINT
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">NULL</span></code> values with <code class="docutils literal"><span class="pre">TRY</span></code>:</p>
<div class="highlight-sql"><div class="highlight"><pre><span></span><span class="k">SELECT</span> <span class="n">TRY</span><span class="p">(</span><span class="k">CAST</span><span class="p">(</span><span class="n">origin_zip</span> <span class="k">AS</span> <span class="nb">BIGINT</span><span class="p">))</span> <span class="k">FROM</span> <span class="n">shipping</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-none"><div class="highlight"><pre><span></span> origin_zip
------------
94131
NULL
94025
08544
(4 rows)
</pre></div>
</div>
<p>Query failure without <code class="docutils literal"><span class="pre">TRY</span></code>:</p>
<div class="highlight-sql"><div class="highlight"><pre><span></span><span class="k">SELECT</span> <span class="n">total_cost</span> <span class="o">/</span> <span class="n">packages</span> <span class="k">AS</span> <span class="n">per_package</span> <span class="k">FROM</span> <span class="n">shipping</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-none"><div class="highlight"><pre><span></span>Query failed: / by zero
</pre></div>
</div>
<p>Default values with <code class="docutils literal"><span class="pre">TRY</span></code> and <code class="docutils literal"><span class="pre">COALESCE</span></code>:</p>
<div class="highlight-sql"><div class="highlight"><pre><span></span><span class="k">SELECT</span> <span class="n">COALESCE</span><span class="p">(</span><span class="n">TRY</span><span class="p">(</span><span class="n">total_cost</span> <span class="o">/</span> <span class="n">packages</span><span class="p">),</span> <span class="mi">0</span><span class="p">)</span> <span class="k">AS</span> <span class="n">per_package</span> <span class="k">FROM</span> <span class="n">shipping</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-none"><div class="highlight"><pre><span></span> per_package
-------------
4
14
0
19
(4 rows)
</pre></div>
</div>
</div>
</div>
</div>
</div>
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