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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!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>7.3. Select Lists</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="queries-table-expressions.html" title="7.2. Table Expressions" /><link rel="next" href="queries-union.html" title="7.4. Combining Queries (UNION, INTERSECT, EXCEPT)" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">7.3. Select Lists</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="queries-table-expressions.html" title="7.2. Table Expressions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="queries.html" title="Chapter 7. Queries">Up</a></td><th width="60%" align="center">Chapter 7. Queries</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="queries-union.html" title="7.4. Combining Queries (UNION, INTERSECT, EXCEPT)">Next</a></td></tr></table><hr /></div><div class="sect1" id="QUERIES-SELECT-LISTS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">7.3. Select Lists <a href="#QUERIES-SELECT-LISTS" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="queries-select-lists.html#QUERIES-SELECT-LIST-ITEMS">7.3.1. Select-List Items</a></span></dt><dt><span class="sect2"><a href="queries-select-lists.html#QUERIES-COLUMN-LABELS">7.3.2. Column Labels</a></span></dt><dt><span class="sect2"><a href="queries-select-lists.html#QUERIES-DISTINCT">7.3.3. <code class="literal">DISTINCT</code></a></span></dt></dl></div><a id="id-1.5.6.7.2" class="indexterm"></a><p>3   As shown in the previous section,4   the table expression in the <code class="command">SELECT</code> command5   constructs an intermediate virtual table by possibly combining6   tables, views, eliminating rows, grouping, etc.  This table is7   finally passed on to processing by the <em class="firstterm">select list</em>.  The select8   list determines which <span class="emphasis"><em>columns</em></span> of the9   intermediate table are actually output.10  </p><div class="sect2" id="QUERIES-SELECT-LIST-ITEMS"><div class="titlepage"><div><div><h3 class="title">7.3.1. Select-List Items <a href="#QUERIES-SELECT-LIST-ITEMS" class="id_link">#</a></h3></div></div></div><a id="id-1.5.6.7.4.2" class="indexterm"></a><p>11    The simplest kind of select list is <code class="literal">*</code> which12    emits all columns that the table expression produces.  Otherwise,13    a select list is a comma-separated list of value expressions (as14    defined in <a class="xref" href="sql-expressions.html" title="4.2. Value Expressions">Section 4.2</a>).  For instance, it15    could be a list of column names:16</p><pre class="programlisting">17SELECT a, b, c FROM ...18</pre><p>19     The columns names <code class="literal">a</code>, <code class="literal">b</code>, and <code class="literal">c</code>20     are either the actual names of the columns of tables referenced21     in the <code class="literal">FROM</code> clause, or the aliases given to them as22     explained in <a class="xref" href="queries-table-expressions.html#QUERIES-TABLE-ALIASES" title="7.2.1.2. Table and Column Aliases">Section 7.2.1.2</a>.  The name23     space available in the select list is the same as in the24     <code class="literal">WHERE</code> clause, unless grouping is used, in which case25     it is the same as in the <code class="literal">HAVING</code> clause.26   </p><p>27    If more than one table has a column of the same name, the table28    name must also be given, as in:29</p><pre class="programlisting">30SELECT tbl1.a, tbl2.a, tbl1.b FROM ...31</pre><p>32    When working with multiple tables, it can also be useful to ask for33    all the columns of a particular table:34</p><pre class="programlisting">35SELECT tbl1.*, tbl2.a FROM ...36</pre><p>37    See <a class="xref" href="rowtypes.html#ROWTYPES-USAGE" title="8.16.5. Using Composite Types in Queries">Section 8.16.5</a> for more about38    the <em class="replaceable"><code>table_name</code></em><code class="literal">.*</code> notation.39   </p><p>40    If an arbitrary value expression is used in the select list, it41    conceptually adds a new virtual column to the returned table.  The42    value expression is evaluated once for each result row, with43    the row's values substituted for any column references.  But the44    expressions in the select list do not have to reference any45    columns in the table expression of the <code class="literal">FROM</code> clause;46    they can be constant arithmetic expressions, for instance.47   </p></div><div class="sect2" id="QUERIES-COLUMN-LABELS"><div class="titlepage"><div><div><h3 class="title">7.3.2. Column Labels <a href="#QUERIES-COLUMN-LABELS" class="id_link">#</a></h3></div></div></div><a id="id-1.5.6.7.5.2" class="indexterm"></a><p>48    The entries in the select list can be assigned names for subsequent49    processing, such as for use in an <code class="literal">ORDER BY</code> clause50    or for display by the client application.  For example:51</p><pre class="programlisting">52SELECT a AS value, b + c AS sum FROM ...53</pre><p>54   </p><p>55    If no output column name is specified using <code class="literal">AS</code>,56    the system assigns a default column name.  For simple column references,57    this is the name of the referenced column.  For function58    calls, this is the name of the function.  For complex expressions,59    the system will generate a generic name.60   </p><p>61    The <code class="literal">AS</code> key word is usually optional, but in some62    cases where the desired column name matches a63    <span class="productname">PostgreSQL</span> key word, you must write64    <code class="literal">AS</code> or double-quote the column name in order to65    avoid ambiguity.66    (<a class="xref" href="sql-keywords-appendix.html" title="Appendix C. SQL Key Words">Appendix C</a> shows which key words67    require <code class="literal">AS</code> to be used as a column label.)68    For example, <code class="literal">FROM</code> is one such key word, so this69    does not work:70</p><pre class="programlisting">71SELECT a from, b + c AS sum FROM ...72</pre><p>73    but either of these do:74</p><pre class="programlisting">75SELECT a AS from, b + c AS sum FROM ...76SELECT a "from", b + c AS sum FROM ...77</pre><p>78    For greatest safety against possible79    future key word additions, it is recommended that you always either80    write <code class="literal">AS</code> or double-quote the output column name.81   </p><div class="note"><h3 class="title">Note</h3><p>82     The naming of output columns here is different from that done in83     the <code class="literal">FROM</code> clause (see <a class="xref" href="queries-table-expressions.html#QUERIES-TABLE-ALIASES" title="7.2.1.2. Table and Column Aliases">Section 7.2.1.2</a>).  It is possible84     to rename the same column twice, but the name assigned in85     the select list is the one that will be passed on.86    </p></div></div><div class="sect2" id="QUERIES-DISTINCT"><div class="titlepage"><div><div><h3 class="title">7.3.3. <code class="literal">DISTINCT</code> <a href="#QUERIES-DISTINCT" class="id_link">#</a></h3></div></div></div><a id="id-1.5.6.7.6.2" class="indexterm"></a><a id="id-1.5.6.7.6.3" class="indexterm"></a><a id="id-1.5.6.7.6.4" class="indexterm"></a><p>87    After the select list has been processed, the result table can88    optionally be subject to the elimination of duplicate rows.  The89    <code class="literal">DISTINCT</code> key word is written directly after90    <code class="literal">SELECT</code> to specify this:91</p><pre class="synopsis">92SELECT DISTINCT <em class="replaceable"><code>select_list</code></em> ...93</pre><p>94    (Instead of <code class="literal">DISTINCT</code> the key word <code class="literal">ALL</code>95    can be used to specify the default behavior of retaining all rows.)96   </p><a id="id-1.5.6.7.6.6" class="indexterm"></a><p>97    Obviously, two rows are considered distinct if they differ in at98    least one column value.  Null values are considered equal in this99    comparison.100   </p><p>101    Alternatively, an arbitrary expression can determine what rows are102    to be considered distinct:103</p><pre class="synopsis">104SELECT DISTINCT ON (<em class="replaceable"><code>expression</code></em> [<span class="optional">, <em class="replaceable"><code>expression</code></em> ...</span>]) <em class="replaceable"><code>select_list</code></em> ...105</pre><p>106    Here <em class="replaceable"><code>expression</code></em> is an arbitrary value107    expression that is evaluated for all rows.  A set of rows for108    which all the expressions are equal are considered duplicates, and109    only the first row of the set is kept in the output.  Note that110    the <span class="quote">“<span class="quote">first row</span>”</span> of a set is unpredictable unless the111    query is sorted on enough columns to guarantee a unique ordering112    of the rows arriving at the <code class="literal">DISTINCT</code> filter.113    (<code class="literal">DISTINCT ON</code> processing occurs after <code class="literal">ORDER114    BY</code> sorting.)115   </p><p>116    The <code class="literal">DISTINCT ON</code> clause is not part of the SQL standard117    and is sometimes considered bad style because of the potentially118    indeterminate nature of its results.  With judicious use of119    <code class="literal">GROUP BY</code> and subqueries in <code class="literal">FROM</code>, this120    construct can be avoided, but it is often the most convenient121    alternative.122   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="queries-table-expressions.html" title="7.2. Table Expressions">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="queries.html" title="Chapter 7. Queries">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="queries-union.html" title="7.4. Combining Queries (UNION, INTERSECT, EXCEPT)">Next</a></td></tr><tr><td width="40%" align="left" valign="top">7.2. Table Expressions </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 7.4. Combining Queries (<code class="literal">UNION</code>, <code class="literal">INTERSECT</code>, <code class="literal">EXCEPT</code>)</td></tr></table></div></body></html>