This results in a much more efficient execution plan because the optimizer can use a MERGE JOIN rather than being forced to use an INNER JOIN as with the CROSS APPLY query above.

what-is-sql-cross-apply-5.sql

SELECT
    soh.SalesOrderID,
    soh.OrderDate,
    sub.*
FROM Sales.SalesOrderHeader soh
JOIN
(
    SELECT
        sod.SalesOrderID,
        SUM (sod.UnitPrice) AS UnitPrice,
        SUM (sod.OrderQty) AS Quantity
    FROM Sales.SalesOrderDetail sod
    GROUP BY sod.SalesOrderID
) sub
ON sub.SalesOrderID = soh.SalesOrderID;