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<div class="section" id="examples-for-using-sql-functions">
<h1>Examples for using SQL functions<a class="headerlink" href="#examples-for-using-sql-functions" title="Permalink to this headline">¶</a></h1>
<p>We assume that you have already created a connection to the PostgreSQL
database, as explained in the <a class="reference internal" href="basic.html"><span class="doc">Basic examples</span></a>:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">pg</span> <span class="k">import</span> <span class="n">DB</span>
<span class="gp">>>> </span><span class="n">db</span> <span class="o">=</span> <span class="n">DB</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">query</span> <span class="o">=</span> <span class="n">db</span><span class="o">.</span><span class="n">query</span>
</pre></div>
</div>
<div class="section" id="creating-sql-functions-on-base-types">
<h2>Creating SQL Functions on Base Types<a class="headerlink" href="#creating-sql-functions-on-base-types" title="Permalink to this headline">¶</a></h2>
<p>A <strong>CREATE FUNCTION</strong> statement lets you create a new function that can be
used in expressions (in SELECT, INSERT, etc.). We will start with functions
that return values of base types.</p>
<p>Let’s create a simple SQL function that takes no arguments and returns 1:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"""CREATE FUNCTION one() RETURNS int4</span>
<span class="gp">... </span><span class="s"> AS 'SELECT 1 as ONE' LANGUAGE SQL"""</span><span class="p">)</span>
</pre></div>
</div>
<p>Functions can be used in any expressions (eg. in the target”list or
qualifications):</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">db</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="s">"SELECT one() AS answer"</span><span class="p">))</span>
<span class="go">answer</span>
<span class="go">------</span>
<span class="go"> 1</span>
<span class="go">(1 row)</span>
</pre></div>
</div>
<p>Here’s how you create a function that takes arguments. The following function
returns the sum of its two arguments:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"""CREATE FUNCTION add_em(int4, int4) RETURNS int4</span>
<span class="gp">... </span><span class="s"> AS $$ SELECT $1 + $2 $$ LANGUAGE SQL"""</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">query</span><span class="p">(</span><span class="s">"SELECT add_em(1, 2) AS answer"</span><span class="p">))</span>
<span class="go">answer</span>
<span class="go">------</span>
<span class="go"> 3</span>
<span class="go">(1 row)</span>
</pre></div>
</div>
</div>
<div class="section" id="creating-sql-functions-on-composite-types">
<h2>Creating SQL Functions on Composite Types<a class="headerlink" href="#creating-sql-functions-on-composite-types" title="Permalink to this headline">¶</a></h2>
<p>It is also possible to create functions that return values of composite types.</p>
<p>Before we create more sophisticated functions, let’s populate an EMP table:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"""CREATE TABLE EMP (</span>
<span class="gp">... </span><span class="s"> name text,</span>
<span class="gp">... </span><span class="s"> salary int4,</span>
<span class="gp">... </span><span class="s"> age f int4,</span>
<span class="gp">... </span><span class="s"> dept varchar(16))"""</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">emps</span> <span class="o">=</span> <span class="p">[</span><span class="s">"'Sam', 1200, 16, 'toy'"</span><span class="p">,</span>
<span class="gp">... </span> <span class="s">"'Claire', 5000, 32, 'shoe'"</span><span class="p">,</span>
<span class="gp">... </span> <span class="s">"'Andy', -1000, 2, 'candy'"</span><span class="p">,</span>
<span class="gp">... </span> <span class="s">"'Bill', 4200, 36, 'shoe'"</span><span class="p">,</span>
<span class="gp">... </span> <span class="s">"'Ginger', 4800, 30, 'candy'"</span><span class="p">]</span>
<span class="gp">>>> </span><span class="k">for</span> <span class="n">emp</span> <span class="ow">in</span> <span class="n">emps</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">query</span><span class="p">(</span><span class="s">"INSERT INTO EMP VALUES (%s)"</span> <span class="o">%</span> <span class="n">emp</span><span class="p">)</span>
</pre></div>
</div>
<p>Every INSERT statement will return a ‘1’ indicating that it has inserted
one row into the EMP table.</p>
<p>The argument of a function can also be a tuple. For instance, <em>double_salary</em>
takes a tuple of the EMP table:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"""CREATE FUNCTION double_salary(EMP) RETURNS int4</span>
<span class="gp">... </span><span class="s"> AS $$ SELECT $1.salary * 2 AS salary $$ LANGUAGE SQL"""</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">query</span><span class="p">(</span><span class="s">"""SELECT name, double_salary(EMP) AS dream</span>
<span class="gp">... </span><span class="s"> FROM EMP WHERE EMP.dept = 'toy'"""</span><span class="p">))</span>
<span class="go">name|dream</span>
<span class="go">----+-----</span>
<span class="go">Sam | 2400</span>
<span class="go">(1 row)</span>
</pre></div>
</div>
<p>The return value of a function can also be a tuple. However, make sure that the
expressions in the target list are in the same order as the columns of EMP:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"""CREATE FUNCTION new_emp() RETURNS EMP AS $$</span>
<span class="gp">... </span><span class="s"> SELECT 'None'::text AS name,</span>
<span class="gp">... </span><span class="s"> 1000 AS salary,</span>
<span class="gp">... </span><span class="s"> 25 AS age,</span>
<span class="gp">... </span><span class="s"> 'None'::varchar(16) AS dept</span>
<span class="gp">... </span><span class="s"> $$ LANGUAGE SQL"""</span><span class="p">)</span>
</pre></div>
</div>
<p>You can then project a column out of resulting the tuple by using the
“function notation” for projection columns (i.e. <code class="docutils literal"><span class="pre">bar(foo)</span></code> is equivalent
to <code class="docutils literal"><span class="pre">foo.bar</span></code>). Note that <code class="docutils literal"><span class="pre">new_emp().name</span></code> isn’t supported:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">query</span><span class="p">(</span><span class="s">"SELECT name(new_emp()) AS nobody"</span><span class="p">))</span>
<span class="go">nobody</span>
<span class="go">------</span>
<span class="go">None</span>
<span class="go">(1 row)</span>
</pre></div>
</div>
<p>Let’s try one more function that returns tuples:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"""CREATE FUNCTION high_pay() RETURNS setof EMP</span>
<span class="gp">... </span><span class="s"> AS 'SELECT * FROM EMP where salary > 1500'</span>
<span class="gp">... </span><span class="s"> LANGUAGE SQL"""</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"SELECT name(high_pay()) AS overpaid"</span><span class="p">)</span>
<span class="go">overpaid</span>
<span class="go">--------</span>
<span class="go">Claire</span>
<span class="go">Bill</span>
<span class="go">Ginger</span>
<span class="go">(3 rows)</span>
</pre></div>
</div>
</div>
<div class="section" id="creating-sql-functions-with-multiple-sql-statements">
<h2>Creating SQL Functions with multiple SQL statements<a class="headerlink" href="#creating-sql-functions-with-multiple-sql-statements" title="Permalink to this headline">¶</a></h2>
<p>You can also create functions that do more than just a SELECT.</p>
<p>You may have noticed that Andy has a negative salary. We’ll create a function
that removes employees with negative salaries:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"SELECT * FROM EMP"</span><span class="p">)</span>
<span class="go"> name |salary|age|dept</span>
<span class="go">------+------+---+-----</span>
<span class="go">Sam | 1200| 16|toy</span>
<span class="go">Claire| 5000| 32|shoe</span>
<span class="go">Andy | -1000| 2|candy</span>
<span class="go">Bill | 4200| 36|shoe</span>
<span class="go">Ginger| 4800| 30|candy</span>
<span class="go">(5 rows)</span>
<span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"""CREATE FUNCTION clean_EMP () RETURNS int4 AS</span>
<span class="gp">... </span><span class="s"> 'DELETE FROM EMP WHERE EMP.salary <= 0;</span>
<span class="gp">... </span><span class="s"> SELECT 1 AS ignore_this'</span>
<span class="gp">... </span><span class="s"> LANGUAGE SQL"""</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"SELECT clean_EMP()"</span><span class="p">)</span>
<span class="go">clean_emp</span>
<span class="go">---------</span>
<span class="go"> 1</span>
<span class="go">(1 row)</span>
<span class="gp">>>> </span><span class="n">query</span><span class="p">(</span><span class="s">"SELECT * FROM EMP"</span><span class="p">)</span>
<span class="go"> name |salary|age|dept</span>
<span class="go">------+------+---+-----</span>
<span class="go">Sam | 1200| 16|toy</span>
<span class="go">Claire| 5000| 32|shoe</span>
<span class="go">Bill | 4200| 36|shoe</span>
<span class="go">Ginger| 4800| 30|candy</span>
<span class="go">(4 rows)</span>
</pre></div>
</div>
</div>
<div class="section" id="remove-functions-that-were-created-in-this-example">
<h2>Remove functions that were created in this example<a class="headerlink" href="#remove-functions-that-were-created-in-this-example" title="Permalink to this headline">¶</a></h2>
<p>We can remove the functions that we have created in this example and the
table EMP, by using the DROP command:</p>
<div class="highlight-default"><div class="highlight"><pre><span class="n">query</span><span class="p">(</span><span class="s">"DROP FUNCTION clean_EMP()"</span><span class="p">)</span>
<span class="n">query</span><span class="p">(</span><span class="s">"DROP FUNCTION high_pay()"</span><span class="p">)</span>
<span class="n">query</span><span class="p">(</span><span class="s">"DROP FUNCTION new_emp()"</span><span class="p">)</span>
<span class="n">query</span><span class="p">(</span><span class="s">"DROP FUNCTION add_em(int4, int4)"</span><span class="p">)</span>
<span class="n">query</span><span class="p">(</span><span class="s">"DROP FUNCTION one()"</span><span class="p">)</span>
<span class="n">query</span><span class="p">(</span><span class="s">"DROP TABLE EMP CASCADE"</span><span class="p">)</span>
</pre></div>
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<h3><a href="../../index.html">Page contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Examples for using SQL functions</a><ul>
<li><a class="reference internal" href="#creating-sql-functions-on-base-types">Creating SQL Functions on Base Types</a></li>
<li><a class="reference internal" href="#creating-sql-functions-on-composite-types">Creating SQL Functions on Composite Types</a></li>
<li><a class="reference internal" href="#creating-sql-functions-with-multiple-sql-statements">Creating SQL Functions with multiple SQL statements</a></li>
<li><a class="reference internal" href="#remove-functions-that-were-created-in-this-example">Remove functions that were created in this example</a></li>
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