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>Joins Between Classes</A
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><P
> Thus far, our queries have only accessed one class at a
time. Queries can access multiple classes at once, or
access the same class in such a way that multiple
instances of the class are being processed at the same
time. A query that accesses multiple instances of the
same or different classes at one time is called a join
query.
As an example, say we wish to find all the records that
are in the temperature range of other records. In
effect, we need to compare the temp_lo and temp_hi
attributes of each EMP instance to the temp_lo and
temp_hi attributes of all other EMP instances.
<BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>This is only a conceptual model. The actual join may
be performed in a more efficient manner, but this is invisible to the user.</P
></BLOCKQUOTE
>
We can do this with the following query:
<PRE
CLASS="PROGRAMLISTING"
>SELECT W1.city, W1.temp_lo, W1.temp_hi,
W2.city, W2.temp_lo, W2.temp_hi
FROM weather W1, weather W2
WHERE W1.temp_lo < W2.temp_lo
AND W1.temp_hi > W2.temp_hi;
+--------------+---------+---------+---------------+---------+---------+
|city | temp_lo | temp_hi | city | temp_lo | temp_hi |
+--------------+---------+---------+---------------+---------+---------+
|San Francisco | 43 | 57 | San Francisco | 46 | 50 |
+--------------+---------+---------+---------------+---------+---------+
|San Francisco | 37 | 54 | San Francisco | 46 | 50 |
+--------------+---------+---------+---------------+---------+---------+</PRE
>
<BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>The semantics of such a join are
that the qualification
is a truth expression defined for the Cartesian product of
the classes indicated in the query. For those instances in
the Cartesian product for which the qualification is true,
<SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
> computes and returns the values specified in the
target list. <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
> <SPAN
CLASS="ACRONYM"
>SQL</SPAN
> does not assign any meaning to
duplicate values in such expressions. This means that <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
>
sometimes recomputes the same target list several times;
this frequently happens when Boolean expressions are connected
with an "or". To remove such duplicates, you must use
the <B
CLASS="COMMAND"
>select distinct</B
> statement.</P
></BLOCKQUOTE
> </P
><P
> In this case, both W1 and W2 are surrogates for an
instance of the class weather, and both range over all
instances of the class. (In the terminology of most
database systems, W1 and W2 are known as <I
CLASS="FIRSTTERM"
>range variables</I
>.)
A query can contain an arbitrary number of
class names and surrogates.</P
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