Monday, March 9, 2009

Use APPLY

Use APPLY
The apply statement was created for the situation where you put multiple inline nested queries in the one statement. For example, take the following statement:
SELECT soh.SalesOrderID,
Quantity=(SELECT TOP 1 (Quantity)
FROM Sales.SalesOrderDetails
WHERE  SalesOrderID = soh.SalesOrderID),
UnitPrice=(SELECT TOP 1 (UnitPrice)
FROM Sales.SalesOrderDetails
WHERE  SalesOrderID = soh.SalesOrderID)
FROM Sales.SalesOrderHeader soh
This performs an extra query, retrieving data from another table using the same criterion. This can now be replaced with the following:

SELECT soh.SalesOrderID, soh.OrderDate, a.*
FROM Sales.SalesOrderHeader soh
CROSS APPLY (
SELECT TOP (1) sod.UnitPrice, sod.Quantity
FROM Sales.SalesOrderDetail sod
WHERE sod.SalesOrderId = soh.SalesOrderId
ORDER BY sod.Quantity DESC
) as a

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