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 employees who have the same manager as Terri Duffy by using a subquery:
USE AdventureWorks; GO SELECT EmployeeID, ManagerID FROM HumanResources.Employee WHERE ManagerID IN (SELECT ManagerID FROM HumanResources.Employee WHERE EmployeeID = 12)
Here is the result set.
EmployeeID ManagerID ----------- ----------- 6 109 12 109 21 109 42 109 140 109 148 109 273 109 (7 row(s) affected)
Or you can use a self-join:
USE AdventureWorks; GO SELECT e1.EmployeeID, e1.ManagerID FROM HumanResources.Employee AS e1 INNER JOIN HumanResources.Employee AS e2 ON e1.ManagerID = e2.ManagerID AND e2.EmployeeID = 12
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 AdventureWorks; GO SELECT e1.EmployeeID, e1.ManagerID FROM HumanResources.Employee AS e1 WHERE e1.ManagerID IN (SELECT e2.ManagerID FROM HumanResources.Employee AS e2 WHERE e2.EmployeeID = 12)
Explicit aliases make it clear that a reference to HumanResources.Employee in the subquery does not mean the same thing as the reference in the outer query.