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>Querying a Class</A
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><P
> The weather class can be queried with normal relational
selection and projection queries. A <SPAN
CLASS="ACRONYM"
>SQL</SPAN
> <B
CLASS="COMMAND"
>select</B
>
statement is used to do this. The statement is divided into
a target list (the part that lists the attributes to be
returned) and a qualification (the part that specifies
any restrictions). For example, to retrieve all the
rows of weather, type:
<PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM WEATHER;</PRE
>
and the output should be:
<PRE
CLASS="PROGRAMLISTING"
>+--------------+---------+---------+------+------------+
|city | temp_lo | temp_hi | prcp | date |
+--------------+---------+---------+------+------------+
|San Francisco | 46 | 50 | 0.25 | 11-27-1994 |
+--------------+---------+---------+------+------------+
|San Francisco | 43 | 57 | 0 | 11-29-1994 |
+--------------+---------+---------+------+------------+
|Hayward | 37 | 54 | | 11-29-1994 |
+--------------+---------+---------+------+------------+</PRE
>
You may specify any arbitrary expressions in the target list. For example, you can do:
<PRE
CLASS="PROGRAMLISTING"
>SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather;</PRE
> </P
><P
> Arbitrary Boolean operators
(<B
CLASS="COMMAND"
>and</B
>, <B
CLASS="COMMAND"
>or</B
> and <B
CLASS="COMMAND"
>not</B
>) are
allowed in the qualification of any query. For example,
<PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM weather
WHERE city = 'San Francisco'
AND prcp > 0.0;
+--------------+---------+---------+------+------------+
|city | temp_lo | temp_hi | prcp | date |
+--------------+---------+---------+------+------------+
|San Francisco | 46 | 50 | 0.25 | 11-27-1994 |
+--------------+---------+---------+------+------------+</PRE
></P
><P
> As a final note, you can specify that the results of a
select can be returned in a <I
CLASS="FIRSTTERM"
>sorted order</I
>
or with <I
CLASS="FIRSTTERM"
>duplicate instances</I
> removed.
<PRE
CLASS="PROGRAMLISTING"
>SELECT DISTINCT city
FROM weather
ORDER BY city;</PRE
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