Question

I’ll use a concrete, but hypothetical, example.

Each Order normally has only one line item:

Orders:

OrderGUID   OrderNumber
=========   ============
{FFB2...}   STL-7442-1      
{3EC6...}   MPT-9931-8A

LineItems:

LineItemGUID   Order ID Quantity   Description
============   ======== ========   =================================
{098FBE3...}   1        7          prefabulated amulite
{1609B09...}   2        32         spurving bearing

But occasionally there will be an order with two line items:

LineItemID   Order ID    Quantity   Description
==========   ========    ========   =================================
{A58A1...}   6,784,329   5          pentametric fan
{0E9BC...}   6,784,329   5          differential girdlespring 

Normally when showing the orders to the user:

SELECT Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM Orders
    INNER JOIN LineItems 
    ON Orders.OrderID = LineItems.OrderID

I want to show the single item on the order. But with this occasional order containing two (or more) items, the orders would appear be duplicated:

OrderNumber   Quantity   Description
===========   ========   ====================
STL-7442-1    7          prefabulated amulite
MPT-9931-8A   32         spurving bearing
KSG-0619-81   5          panametric fan
KSG-0619-81   5          differential girdlespring

What I really want is to have SQL Server just pick one, as it will be good enough:

OrderNumber   Quantity   Description
===========   ========   ====================
STL-7442-1    7          prefabulated amulite
MPT-9931-8A   32         differential girdlespring
KSG-0619-81   5          panametric fan

If I get adventurous, I might show the user, an ellipsis to indicate that there’s more than one:

OrderNumber   Quantity   Description
===========   ========   ====================
STL-7442-1    7          prefabulated amulite
MPT-9931-8A   32         differential girdlespring
KSG-0619-81   5          panametric fan, ...

So the question is how to either

• eliminate “duplicate” rows
• only join to one of the rows, to avoid duplication

First attempt

My first naive attempt was to only join to the “TOP 1” line items:

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

But that gives the error:

The column or prefix ‘Orders’ does not
match with a table name or alias name
used in the query.

Presumably because the inner select doesn’t see the outer table.

Answer 1

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
CROSS APPLY
        (
        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