diff options
| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-04 12:19:15 +0000 |
|---|---|---|
| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-04 12:19:15 +0000 |
| commit | 6eb9c5a5657d1fe77b55cc261450f3538d35a94d (patch) | |
| tree | 657d8194422a5daccecfd42d654b8a245ef7b4c8 /doc/src/sgml/html/tutorial-select.html | |
| parent | Initial commit. (diff) | |
| download | postgresql-13-upstream.tar.xz postgresql-13-upstream.zip | |
Adding upstream version 13.4.upstream/13.4upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'doc/src/sgml/html/tutorial-select.html')
| -rw-r--r-- | doc/src/sgml/html/tutorial-select.html | 142 |
1 files changed, 142 insertions, 0 deletions
diff --git a/doc/src/sgml/html/tutorial-select.html b/doc/src/sgml/html/tutorial-select.html new file mode 100644 index 0000000..2a3c663 --- /dev/null +++ b/doc/src/sgml/html/tutorial-select.html @@ -0,0 +1,142 @@ +<?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>2.5. Querying a Table</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 V1.79.1" /><link rel="prev" href="tutorial-populate.html" title="2.4. Populating a Table With Rows" /><link rel="next" href="tutorial-join.html" title="2.6. Joins Between Tables" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">2.5. Querying a Table</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tutorial-populate.html" title="2.4. Populating a Table With Rows">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 xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym> Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="tutorial-join.html" title="2.6. Joins Between Tables">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="TUTORIAL-SELECT"><div class="titlepage"><div><div><h2 class="title" style="clear: both">2.5. Querying a Table</h2></div></div></div><p> + <a id="id-1.4.4.6.2.1" class="indexterm"></a> + <a id="id-1.4.4.6.2.2" class="indexterm"></a> + + To retrieve data from a table, the table is + <em class="firstterm">queried</em>. An <acronym class="acronym">SQL</acronym> + <code class="command">SELECT</code> statement is used to do this. The + statement is divided into a select list (the part that lists the + columns to be returned), a table list (the part that lists the + tables from which to retrieve the data), and an optional + qualification (the part that specifies any restrictions). For + example, to retrieve all the rows of table + <code class="structname">weather</code>, type: +</p><pre class="programlisting"> +SELECT * FROM weather; +</pre><p> + Here <code class="literal">*</code> is a shorthand for <span class="quote">“<span class="quote">all columns</span>”</span>. + <a href="#ftn.id-1.4.4.6.2.10" class="footnote"><sup class="footnote" id="id-1.4.4.6.2.10">[2]</sup></a> + So the same result would be had with: +</p><pre class="programlisting"> +SELECT city, temp_lo, temp_hi, prcp, date FROM weather; +</pre><p> + + The output should be: + +</p><pre class="screen"> + city | temp_lo | temp_hi | prcp | date +---------------+---------+---------+------+------------ + San Francisco | 46 | 50 | 0.25 | 1994-11-27 + San Francisco | 43 | 57 | 0 | 1994-11-29 + Hayward | 37 | 54 | | 1994-11-29 +(3 rows) +</pre><p> + </p><p> + You can write expressions, not just simple column references, in the + select list. For example, you can do: +</p><pre class="programlisting"> +SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather; +</pre><p> + This should give: +</p><pre class="screen"> + city | temp_avg | date +---------------+----------+------------ + San Francisco | 48 | 1994-11-27 + San Francisco | 50 | 1994-11-29 + Hayward | 45 | 1994-11-29 +(3 rows) +</pre><p> + Notice how the <code class="literal">AS</code> clause is used to relabel the + output column. (The <code class="literal">AS</code> clause is optional.) + </p><p> + A query can be <span class="quote">“<span class="quote">qualified</span>”</span> by adding a <code class="literal">WHERE</code> + clause that specifies which rows are wanted. The <code class="literal">WHERE</code> + clause contains a Boolean (truth value) expression, and only rows for + which the Boolean expression is true are returned. The usual + Boolean operators (<code class="literal">AND</code>, + <code class="literal">OR</code>, and <code class="literal">NOT</code>) are allowed in + the qualification. For example, the following + retrieves the weather of San Francisco on rainy days: + +</p><pre class="programlisting"> +SELECT * FROM weather + WHERE city = 'San Francisco' AND prcp > 0.0; +</pre><p> + Result: +</p><pre class="screen"> + city | temp_lo | temp_hi | prcp | date +---------------+---------+---------+------+------------ + San Francisco | 46 | 50 | 0.25 | 1994-11-27 +(1 row) +</pre><p> + </p><p> + <a id="id-1.4.4.6.5.1" class="indexterm"></a> + + You can request that the results of a query + be returned in sorted order: + +</p><pre class="programlisting"> +SELECT * FROM weather + ORDER BY city; +</pre><p> + +</p><pre class="screen"> + city | temp_lo | temp_hi | prcp | date +---------------+---------+---------+------+------------ + Hayward | 37 | 54 | | 1994-11-29 + San Francisco | 43 | 57 | 0 | 1994-11-29 + San Francisco | 46 | 50 | 0.25 | 1994-11-27 +</pre><p> + + In this example, the sort order isn't fully specified, and so you + might get the San Francisco rows in either order. But you'd always + get the results shown above if you do: + +</p><pre class="programlisting"> +SELECT * FROM weather + ORDER BY city, temp_lo; +</pre><p> + </p><p> + <a id="id-1.4.4.6.6.1" class="indexterm"></a> + <a id="id-1.4.4.6.6.2" class="indexterm"></a> + + You can request that duplicate rows be removed from the result of + a query: + +</p><pre class="programlisting"> +SELECT DISTINCT city + FROM weather; +</pre><p> + +</p><pre class="screen"> + city +--------------- + Hayward + San Francisco +(2 rows) +</pre><p> + + Here again, the result row ordering might vary. + You can ensure consistent results by using <code class="literal">DISTINCT</code> and + <code class="literal">ORDER BY</code> together: + <a href="#ftn.id-1.4.4.6.6.7" class="footnote"><sup class="footnote" id="id-1.4.4.6.6.7">[3]</sup></a> + +</p><pre class="programlisting"> +SELECT DISTINCT city + FROM weather + ORDER BY city; +</pre><p> + </p><div class="footnotes"><br /><hr style="width:100; text-align:left;margin-left: 0" /><div id="ftn.id-1.4.4.6.2.10" class="footnote"><p><a href="#id-1.4.4.6.2.10" class="para"><sup class="para">[2] </sup></a> + While <code class="literal">SELECT *</code> is useful for off-the-cuff + queries, it is widely considered bad style in production code, + since adding a column to the table would change the results. + </p></div><div id="ftn.id-1.4.4.6.6.7" class="footnote"><p><a href="#id-1.4.4.6.6.7" class="para"><sup class="para">[3] </sup></a> + In some database systems, including older versions of + <span class="productname">PostgreSQL</span>, the implementation of + <code class="literal">DISTINCT</code> automatically orders the rows and + so <code class="literal">ORDER BY</code> is unnecessary. But this is not + required by the SQL standard, and current + <span class="productname">PostgreSQL</span> does not guarantee that + <code class="literal">DISTINCT</code> causes the rows to be ordered. + </p></div></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tutorial-populate.html" title="2.4. Populating a Table With Rows">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-join.html" title="2.6. Joins Between Tables">Next</a></td></tr><tr><td width="40%" align="left" valign="top">2.4. Populating a Table With Rows </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 2.6. Joins Between Tables</td></tr></table></div></body></html>
\ No newline at end of file |