Cross-table-Query

• Sometimes a single table does not contain all desired attributes.

Cross table

The crossing of tables is the cartesian product of them. Equivalent to querying from a (temporary) table A×B, the cartesian product of A and B.

temp=city x country
SELECT city FROM temp WHERE city.country_id=country.country_id AND country='China'

A table times itself

• 对于一部分相同但是仍有部份不同需要找出时

SELECT a1.city_id
FROM address AS a1, address AS a2
WHERE a1.city_id = a2.city_id AND
 a1.address_id <> a2.address_id