Apply Operator

CROSS APPLY = T-SQL Inner Join

SELECT *
FROM Department D
CROSS APPLY (
    SELECT *
    FROM Employee E
    WHERE E.DepartmentID = D.DepartmentID
) A

-- The result sets from these would have been identical

SELECT *
FROM Department D
INNER JOIN Employee E
  ON D.DepartmentID = E.DepartmentID

OUTER APPLY = T-SQL Left Join

SELECT *
FROM Department D
OUTER APPLY (
    SELECT *
    FROM Employee E
    WHERE E.DepartmentID = D.DepartmentID
) A


-- The result sets from these would have been identical

SELECT *
FROM Department D
LEFT OUTER JOIN Employee E
  ON D.DepartmentID = E.DepartmentID

Great uses for Apply

• SO Examples for apply
• Examples of main use cases