Subqueries with Aliases
Many statements in which the subquery and the outer query refer to the same table can be stated as self-joins (joining a table to itself). For example, you can find addresses of employees from a particular state using a subquery:
USE AdventureWorks2008R2; GO SELECT StateProvinceID, AddressID FROM Person.Address WHERE AddressID IN (SELECT AddressID FROM Person.Address WHERE StateProvinceID = 39)
Here is the result set.
(4 row(s) affected)
Or you can use a self-join:
USE AdventureWorks2008R2; GO SELECT e1.StateProvinceID, e1.AddressID FROM Person.Address AS e1 INNER JOIN Person.Address AS e2 ON e1.AddressID = e2.AddressID AND e2.StateProvinceID = 39;
Table aliases are required because the table being joined to itself appears in two different roles. Aliases can also be used in nested queries that refer to the same table in an inner and outer query.
USE AdventureWorks2008R2; GO SELECT e1.StateProvinceID, e1.AddressID FROM Person.Address AS e1 WHERE e1.AddressID IN (SELECT e2.AddressID FROM Person.Address AS e2 WHERE e2.StateProvinceID = 39)
Explicit aliases make it clear that a reference to Person.Address in the subquery does not mean the same thing as the reference in the outer query.