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><H1
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><A
NAME="TUTORIAL-SELECT"
>2.5. Querying a Table</A
></H1
><P
>
To retrieve data from a table, the table is
<I
CLASS="FIRSTTERM"
>queried</I
>. An <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>
<TT
CLASS="COMMAND"
>SELECT</TT
> 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
<TT
CLASS="STRUCTNAME"
>weather</TT
>, type:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM weather;</PRE
><P>
Here <TT
CLASS="LITERAL"
>*</TT
> is a shorthand for <SPAN
CLASS="QUOTE"
>"all columns"</SPAN
>.
<A
NAME="AEN721"
HREF="#FTN.AEN721"
><SPAN
CLASS="footnote"
>[1]</SPAN
></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 <TT
CLASS="LITERAL"
>AS</TT
> clause is used to relabel the
output column. (The <TT
CLASS="LITERAL"
>AS</TT
> clause is optional.)
</P
><P
> A query can be <SPAN
CLASS="QUOTE"
>"qualified"</SPAN
> by adding a <TT
CLASS="LITERAL"
>WHERE</TT
>
clause that specifies which rows are wanted. The <TT
CLASS="LITERAL"
>WHERE</TT
>
clause contains a Boolean (truth value) expression, and only rows for
which the Boolean expression is true are returned. The usual
Boolean operators (<TT
CLASS="LITERAL"
>AND</TT
>,
<TT
CLASS="LITERAL"
>OR</TT
>, and <TT
CLASS="LITERAL"
>NOT</TT
>) 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
>
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
>
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 <TT
CLASS="LITERAL"
>DISTINCT</TT
> and
<TT
CLASS="LITERAL"
>ORDER BY</TT
> together:
<A
NAME="AEN755"
HREF="#FTN.AEN755"
><SPAN
CLASS="footnote"
>[2]</SPAN
></A
>
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT DISTINCT city
FROM weather
ORDER BY city;</PRE
><P>
</P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
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><TD
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WIDTH="5%"
><A
NAME="FTN.AEN721"
HREF="tutorial-select.html#AEN721"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
> While <TT
CLASS="LITERAL"
>SELECT *</TT
> 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
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN755"
HREF="tutorial-select.html#AEN755"
><SPAN
CLASS="footnote"
>[2]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
> In some database systems, including older versions of
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, the implementation of
<TT
CLASS="LITERAL"
>DISTINCT</TT
> automatically orders the rows and
so <TT
CLASS="LITERAL"
>ORDER BY</TT
> is unnecessary. But this is not
required by the SQL standard, and current
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does not guarantee that
<TT
CLASS="LITERAL"
>DISTINCT</TT
> causes the rows to be ordered.
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