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diff --git a/doc/src/sgml/html/tutorial-agg.html b/doc/src/sgml/html/tutorial-agg.html new file mode 100644 index 0000000..b89fd78 --- /dev/null +++ b/doc/src/sgml/html/tutorial-agg.html @@ -0,0 +1,172 @@ +<?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>2.7. Aggregate Functions</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="tutorial-join.html" title="2.6. Joins Between Tables" /><link rel="next" href="tutorial-update.html" title="2.8. Updates" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">2.7. Aggregate Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tutorial-join.html" title="2.6. Joins Between Tables">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="tutorial-sql.html" title="Chapter 2. The SQL Language">Up</a></td><th width="60%" align="center">Chapter 2. The <acronym class="acronym">SQL</acronym> Language</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="tutorial-update.html" title="2.8. Updates">Next</a></td></tr></table><hr /></div><div class="sect1" id="TUTORIAL-AGG"><div class="titlepage"><div><div><h2 class="title" style="clear: both">2.7. Aggregate Functions</h2></div></div></div><a id="id-1.4.4.8.2" class="indexterm"></a><p> + Like most other relational database products, + <span class="productname">PostgreSQL</span> supports + <em class="firstterm">aggregate functions</em>. + An aggregate function computes a single result from multiple input rows. + For example, there are aggregates to compute the + <code class="function">count</code>, <code class="function">sum</code>, + <code class="function">avg</code> (average), <code class="function">max</code> (maximum) and + <code class="function">min</code> (minimum) over a set of rows. + </p><p> + As an example, we can find the highest low-temperature reading anywhere + with: + +</p><pre class="programlisting"> +SELECT max(temp_lo) FROM weather; +</pre><p> + +</p><pre class="screen"> + max +----- + 46 +(1 row) +</pre><p> + </p><p> + <a id="id-1.4.4.8.5.1" class="indexterm"></a> + + If we wanted to know what city (or cities) that reading occurred in, + we might try: + +</p><pre class="programlisting"> +SELECT city FROM weather WHERE temp_lo = max(temp_lo); <em class="lineannotation"><span class="lineannotation">WRONG</span></em> +</pre><p> + + but this will not work since the aggregate + <code class="function">max</code> cannot be used in the + <code class="literal">WHERE</code> clause. (This restriction exists because + the <code class="literal">WHERE</code> clause determines which rows will be + included in the aggregate calculation; so obviously it has to be evaluated + before aggregate functions are computed.) + However, as is often the case + the query can be restated to accomplish the desired result, here + by using a <em class="firstterm">subquery</em>: + +</p><pre class="programlisting"> +SELECT city FROM weather + WHERE temp_lo = (SELECT max(temp_lo) FROM weather); +</pre><p> + +</p><pre class="screen"> + city +--------------- + San Francisco +(1 row) +</pre><p> + + This is OK because the subquery is an independent computation + that computes its own aggregate separately from what is happening + in the outer query. + </p><p> + <a id="id-1.4.4.8.6.1" class="indexterm"></a> + <a id="id-1.4.4.8.6.2" class="indexterm"></a> + + Aggregates are also very useful in combination with <code class="literal">GROUP + BY</code> clauses. For example, we can get the number of readings + and the maximum low temperature observed in each city with: + +</p><pre class="programlisting"> +SELECT city, count(*), max(temp_lo) + FROM weather + GROUP BY city; +</pre><p> + +</p><pre class="screen"> + city | count | max +---------------+-------+----- + Hayward | 1 | 37 + San Francisco | 2 | 46 +(2 rows) +</pre><p> + + which gives us one output row per city. Each aggregate result is + computed over the table rows matching that city. + We can filter these grouped + rows using <code class="literal">HAVING</code>: + +</p><pre class="programlisting"> +SELECT city, count(*), max(temp_lo) + FROM weather + GROUP BY city + HAVING max(temp_lo) < 40; +</pre><p> + +</p><pre class="screen"> + city | count | max +---------+-------+----- + Hayward | 1 | 37 +(1 row) +</pre><p> + + which gives us the same results for only the cities that have all + <code class="structfield">temp_lo</code> values below 40. Finally, if we only care about + cities whose + names begin with <span class="quote">“<span class="quote"><code class="literal">S</code></span>”</span>, we might do: + +</p><pre class="programlisting"> +SELECT city, count(*), max(temp_lo) + FROM weather + WHERE city LIKE 'S%' -- <span id="co.tutorial-agg-like"></span>(1) + GROUP BY city; +</pre><p> + +</p><pre class="screen"> + city | count | max +---------------+-------+----- + San Francisco | 2 | 46 +(1 row) +</pre><p> + </p><div class="calloutlist"><table border="0" summary="Callout list"><tr><td width="5%" valign="top" align="left"><p><a href="#co.tutorial-agg-like">(1)</a> </p></td><td valign="top" align="left"><p> + The <code class="literal">LIKE</code> operator does pattern matching and + is explained in <a class="xref" href="functions-matching.html" title="9.7. Pattern Matching">Section 9.7</a>. + </p></td></tr></table></div><p> + </p><p> + It is important to understand the interaction between aggregates and + <acronym class="acronym">SQL</acronym>'s <code class="literal">WHERE</code> and <code class="literal">HAVING</code> clauses. + The fundamental difference between <code class="literal">WHERE</code> and + <code class="literal">HAVING</code> is this: <code class="literal">WHERE</code> selects + input rows before groups and aggregates are computed (thus, it controls + which rows go into the aggregate computation), whereas + <code class="literal">HAVING</code> selects group rows after groups and + aggregates are computed. Thus, the + <code class="literal">WHERE</code> clause must not contain aggregate functions; + it makes no sense to try to use an aggregate to determine which rows + will be inputs to the aggregates. On the other hand, the + <code class="literal">HAVING</code> clause always contains aggregate functions. + (Strictly speaking, you are allowed to write a <code class="literal">HAVING</code> + clause that doesn't use aggregates, but it's seldom useful. The same + condition could be used more efficiently at the <code class="literal">WHERE</code> + stage.) + </p><p> + In the previous example, we can apply the city name restriction in + <code class="literal">WHERE</code>, since it needs no aggregate. This is + more efficient than adding the restriction to <code class="literal">HAVING</code>, + because we avoid doing the grouping and aggregate calculations + for all rows that fail the <code class="literal">WHERE</code> check. + </p><p> + Another way to select the rows that go into an aggregate + computation is to use <code class="literal">FILTER</code>, which is a + per-aggregate option: + +</p><pre class="programlisting"> +SELECT city, count(*) FILTER (WHERE temp_lo < 45), max(temp_lo) + FROM weather + GROUP BY city; +</pre><p> + +</p><pre class="screen"> + city | count | max +---------------+-------+----- + Hayward | 1 | 37 + San Francisco | 1 | 46 +(2 rows) +</pre><p> + + <code class="literal">FILTER</code> is much like <code class="literal">WHERE</code>, + except that it removes rows only from the input of the particular + aggregate function that it is attached to. + Here, the <code class="literal">count</code> aggregate counts only + rows with <code class="literal">temp_lo</code> below 45; but the + <code class="literal">max</code> aggregate is still applied to all rows, + so it still finds the reading of 46. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tutorial-join.html" title="2.6. Joins Between Tables">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="tutorial-sql.html" title="Chapter 2. The SQL Language">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tutorial-update.html" title="2.8. Updates">Next</a></td></tr><tr><td width="40%" align="left" valign="top">2.6. Joins Between Tables </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"> 2.8. Updates</td></tr></table></div></body></html>
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