CROSS APPLY and OUTER APPLY clause in SQL-SERVER 2005

CROSS APPLY and OUTER APPLY clause in SQL-SERVER 2005
I will be introducing you with magical feature of Microsoft SQL-Server 2005, called CROSS APPLY and OUTER APPLY. If you want to get top 2 or 3 maximum inventory quantity of each product or all product of AdventureWorks database then it is bit difficult in SQL-Server 2000 as it doesn’t support CROSS APPLY or OUTER APPLY. You can use UDF or sub query for CROSS APPY or OUTER APPLY.
BTW, you can consider CROSS APPLY clause as INNER APPLY also as it will use outer (main) query as an input of subquery or function and will return the result set. In CROSS APPLY we will be getting full set of left side query (main query) and its corresponding value from right side query or function, if it is not available in right side query or function, it will return NULL.
First let us see use of CROSS APPLY:
We are going to create one function which will return top row from production.productinventory table of AdventureWorks database based on supplied productID and row number.
CREATE FUNCTION dbo.fn_GetMax_ProductItem(@Productid AS int, @rownum AS INT)
RETURNS TABLE
AS
RETURN
SELECT TOP(@rownum) *
FROM Production.ProductInventory
WHERE ProductID = @Productid
ORDER BY Quantity DESC
GO
You can run above UDF and test it with following query.
select * from dbo.fn_GetMax_ProductItem(1,2)
It will return top two row of productID 1.
Note: I strongly recommend not using “*” in query in live production environment due to performance issue, this is ok in this example.
As function is ready, we will start using that function in our CROSS APPLY example.
SELECT
p.ProductID,
p.Name,
p.ProductNumber,
pmi.Quantity,
pmi.locationID
FROM
Production.Product AS p
CROSS APPLY
dbo.fn_GetMax_ProductItem(p.productid, 2) AS pmi
WHERE
p.ProductID in (1,2,3,531,706)
ORDER BY
p.productid ASC
As soon as you run above query, you will get 8 rows. Two rows for each productID (1,2,3,531). You will not get any row for ProductID 706 as it is not available in Production.Product table. This proves that CROSS APPLY clause works like INNER APPLY.
Now let us tweak above query a bit with OUTER APPLY instead of CROSS APPLY.
SELECT
p.ProductID,
p.Name,
p.ProductNumber,
pmi.Quantity,
pmi.locationID
FROM
Production.Product AS p
OUTER APPLY
dbo.fn_GetMax_ProductItem(p.productid, 2) AS pmi
WHERE
p.ProductID in (1,2,3,531,706)
ORDER BY
p.productid ASC
This time you will get records for 706 productID as well but it will come with NULL in Quantity and LocationID.
Reference: Ritesh Shah
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