How to Eliminate Duplicate Order Numbers When Joining Tables with Multiple Line Items?

Avoiding Duplicate Order Numbers in Multi-Line Item Joins

Database joins involving tables with multiple line items per order often lead to duplicate order numbers in the results. This article explores efficient solutions to retrieve only one record per unique order.

Initial Approach and Its Shortcomings

A simple approach might involve selecting only the "TOP 1" line item. However, this fails due to limitations in directly accessing the outer table from within the inner select statement.

Efficient Solution: CROSS APPLY

The CROSS APPLY operator provides an elegant solution:

<code class="language-sql">SELECT  Orders.OrderNumber, LineItems2.Quantity, LineItems2.Description
FROM    Orders
CROSS APPLY
        (
        SELECT  TOP 1 LineItems.Quantity, LineItems.Description
        FROM    LineItems
        WHERE   LineItems.OrderID = Orders.OrderID
        ) LineItems2</code>

CROSS APPLY efficiently retrieves the first line item for each order, preventing duplicate order numbers.

Alternative for Older SQL Server Versions: INNER JOIN

For SQL Server versions before 2005, an INNER JOIN offers a comparable solution:

<code class="language-sql">SELECT   Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM     Orders
JOIN     LineItems
ON       LineItems.LineItemGUID =
         (
         SELECT  TOP 1 LineItemGUID 
         FROM    LineItems
         WHERE   OrderID = Orders.OrderID
         )</code>

Ensuring Deterministic Results

Crucially, TOP 1 without an ORDER BY clause is non-deterministic. The selected line item might vary between query executions, even with unchanged data. Always include an ORDER BY clause within the inner query to guarantee consistent results.

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