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><H1
CLASS="SECT1"
><A
NAME="QUERIES-SELECT-LISTS"
>7.3. Select Lists</A
></H1
><P
> As shown in the previous section,
the table expression in the <TT
CLASS="COMMAND"
>SELECT</TT
> command
constructs an intermediate virtual table by possibly combining
tables, views, eliminating rows, grouping, etc. This table is
finally passed on to processing by the <I
CLASS="FIRSTTERM"
>select list</I
>. The select
list determines which <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>columns</I
></SPAN
> of the
intermediate table are actually output.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="QUERIES-SELECT-LIST-ITEMS"
>7.3.1. Select-List Items</A
></H2
><P
> The simplest kind of select list is <TT
CLASS="LITERAL"
>*</TT
> which
emits all columns that the table expression produces. Otherwise,
a select list is a comma-separated list of value expressions (as
defined in <A
HREF="sql-expressions.html"
>Section 4.2</A
>). For instance, it
could be a list of column names:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a, b, c FROM ...</PRE
><P>
The columns names <TT
CLASS="LITERAL"
>a</TT
>, <TT
CLASS="LITERAL"
>b</TT
>, and <TT
CLASS="LITERAL"
>c</TT
>
are either the actual names of the columns of tables referenced
in the <TT
CLASS="LITERAL"
>FROM</TT
> clause, or the aliases given to them as
explained in <A
HREF="queries-table-expressions.html#QUERIES-TABLE-ALIASES"
>Section 7.2.1.2</A
>. The name
space available in the select list is the same as in the
<TT
CLASS="LITERAL"
>WHERE</TT
> clause, unless grouping is used, in which case
it is the same as in the <TT
CLASS="LITERAL"
>HAVING</TT
> clause.
</P
><P
> If more than one table has a column of the same name, the table
name must also be given, as in:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT tbl1.a, tbl2.a, tbl1.b FROM ...</PRE
><P>
When working with multiple tables, it can also be useful to ask for
all the columns of a particular table:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT tbl1.*, tbl2.a FROM ...</PRE
><P>
See <A
HREF="rowtypes.html#ROWTYPES-USAGE"
>Section 8.16.5</A
> for more about
the <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
><TT
CLASS="LITERAL"
>.*</TT
> notation.
</P
><P
> If an arbitrary value expression is used in the select list, it
conceptually adds a new virtual column to the returned table. The
value expression is evaluated once for each result row, with
the row's values substituted for any column references. But the
expressions in the select list do not have to reference any
columns in the table expression of the <TT
CLASS="LITERAL"
>FROM</TT
> clause;
they can be constant arithmetic expressions, for instance.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="QUERIES-COLUMN-LABELS"
>7.3.2. Column Labels</A
></H2
><P
> The entries in the select list can be assigned names for subsequent
processing, such as for use in an <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause
or for display by the client application. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a AS value, b + c AS sum FROM ...</PRE
><P>
</P
><P
> If no output column name is specified using <TT
CLASS="LITERAL"
>AS</TT
>,
the system assigns a default column name. For simple column references,
this is the name of the referenced column. For function
calls, this is the name of the function. For complex expressions,
the system will generate a generic name.
</P
><P
> The <TT
CLASS="LITERAL"
>AS</TT
> keyword is optional, but only if the new column
name does not match any
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> keyword (see <A
HREF="sql-keywords-appendix.html"
>Appendix C</A
>). To avoid an accidental match to
a keyword, you can double-quote the column name. For example,
<TT
CLASS="LITERAL"
>VALUE</TT
> is a keyword, so this does not work:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a value, b + c AS sum FROM ...</PRE
><P>
but this does:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a "value", b + c AS sum FROM ...</PRE
><P>
For protection against possible
future keyword additions, it is recommended that you always either
write <TT
CLASS="LITERAL"
>AS</TT
> or double-quote the output column name.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> The naming of output columns here is different from that done in
the <TT
CLASS="LITERAL"
>FROM</TT
> clause (see <A
HREF="queries-table-expressions.html#QUERIES-TABLE-ALIASES"
>Section 7.2.1.2</A
>). It is possible
to rename the same column twice, but the name assigned in
the select list is the one that will be passed on.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="QUERIES-DISTINCT"
>7.3.3. <TT
CLASS="LITERAL"
>DISTINCT</TT
></A
></H2
><P
> After the select list has been processed, the result table can
optionally be subject to the elimination of duplicate rows. The
<TT
CLASS="LITERAL"
>DISTINCT</TT
> key word is written directly after
<TT
CLASS="LITERAL"
>SELECT</TT
> to specify this:
</P><PRE
CLASS="SYNOPSIS"
>SELECT DISTINCT <TT
CLASS="REPLACEABLE"
><I
>select_list</I
></TT
> ...</PRE
><P>
(Instead of <TT
CLASS="LITERAL"
>DISTINCT</TT
> the key word <TT
CLASS="LITERAL"
>ALL</TT
>
can be used to specify the default behavior of retaining all rows.)
</P
><P
> Obviously, two rows are considered distinct if they differ in at
least one column value. Null values are considered equal in this
comparison.
</P
><P
> Alternatively, an arbitrary expression can determine what rows are
to be considered distinct:
</P><PRE
CLASS="SYNOPSIS"
>SELECT DISTINCT ON (<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> ...</SPAN
>]) <TT
CLASS="REPLACEABLE"
><I
>select_list</I
></TT
> ...</PRE
><P>
Here <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> is an arbitrary value
expression that is evaluated for all rows. A set of rows for
which all the expressions are equal are considered duplicates, and
only the first row of the set is kept in the output. Note that
the <SPAN
CLASS="QUOTE"
>"first row"</SPAN
> of a set is unpredictable unless the
query is sorted on enough columns to guarantee a unique ordering
of the rows arriving at the <TT
CLASS="LITERAL"
>DISTINCT</TT
> filter.
(<TT
CLASS="LITERAL"
>DISTINCT ON</TT
> processing occurs after <TT
CLASS="LITERAL"
>ORDER
BY</TT
> sorting.)
</P
><P
> The <TT
CLASS="LITERAL"
>DISTINCT ON</TT
> clause is not part of the SQL standard
and is sometimes considered bad style because of the potentially
indeterminate nature of its results. With judicious use of
<TT
CLASS="LITERAL"
>GROUP BY</TT
> and subqueries in <TT
CLASS="LITERAL"
>FROM</TT
>, this
construct can be avoided, but it is often the most convenient
alternative.
</P
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