How to Join to first row

SELECT   Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM     Orders
JOIN     LineItems
ON       LineItems.LineItemGUID =
         SELECT  TOP 1 LineItemGUID 
         FROM    LineItems
         WHERE   OrderID = Orders.OrderID

In SQL Server 2005 and above, you could just replace INNER JOIN with CROSS APPLY:

SELECT  Orders.OrderNumber, LineItems2.Quantity, LineItems2.Description
FROM    Orders
        SELECT  TOP 1 LineItems.Quantity, LineItems.Description
        FROM    LineItems
        WHERE   LineItems.OrderID = Orders.OrderID
        ) LineItems2

Please note that TOP 1 without ORDER BY is not deterministic: this query you will get you one line item per order, but it is not defined which one will it be.

Multiple invocations of the query can give you different line items for the same order, even if the underlying did not change.

If you want deterministic order, you should add an ORDER BY clause to the innermost query.

Example sqlfiddle

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