Some uses for APPLY
are…
1) Top N per group queries (can be more efficient for some cardinalities)
SELECT pr.name,
pa.name
FROM sys.procedures pr
OUTER APPLY (SELECT TOP 2 *
FROM sys.parameters pa
WHERE pa.object_id = pr.object_id
ORDER BY pr.name) pa
ORDER BY pr.name,
pa.name
2) Calling a Table Valued Function for each row in the outer query
SELECT *
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle)
SELECT number,
doubled_number,
doubled_number_plus_one
FROM master..spt_values
CROSS APPLY (SELECT 2 * CAST(number AS BIGINT)) CA1(doubled_number)
CROSS APPLY (SELECT doubled_number + 1) CA2(doubled_number_plus_one)
4) Unpivoting more than one group of columns
Assumes 1NF violating table structure….
CREATE TABLE T
(
Id INT PRIMARY KEY,
Foo1 INT, Foo2 INT, Foo3 INT,
Bar1 INT, Bar2 INT, Bar3 INT
);
Example using 2008+ VALUES
syntax.
SELECT Id,
Foo,
Bar
FROM T
CROSS APPLY (VALUES(Foo1, Bar1),
(Foo2, Bar2),
(Foo3, Bar3)) V(Foo, Bar);
In 2005 UNION ALL
can be used instead.
SELECT Id,
Foo,
Bar
FROM T
CROSS APPLY (SELECT Foo1, Bar1
UNION ALL
SELECT Foo2, Bar2
UNION ALL
SELECT Foo3, Bar3) V(Foo, Bar);