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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.
Note: This is only a conceptual model. The actual join may be performed in a more efficient manner, but this is invisible to the user.
SELECT W1.city, W1.temp_lo AS low, W1.temp_hi AS high,
W2.city, W2.temp_lo AS low, W2.temp_hi AS high
FROM weather W1, weather W2
WHERE W1.temp_lo < W2.temp_lo
AND W1.temp_hi > W2.temp_hi;
+--------------+-----+------+---------------+-----+------+
|city | low | high | city | low | high |
+--------------+-----+------+---------------+-----+------+
|San Francisco | 43 | 57 | San Francisco | 46 | 50 |
+--------------+-----+------+---------------+-----+------+
|San Francisco | 37 | 54 | San Francisco | 46 | 50 |
+--------------+-----+------+---------------+-----+------+
Note: 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, Postgres computes and returns the values specified in the target list. Postgres SQL does not assign any meaning to duplicate values in such expressions. This means that Postgres 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 select distinct statement.
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 range variables.) A query can contain an arbitrary number of class names and surrogates.