How Can I Efficiently Join to Only the First Row of a Related Table in SQL?

Selecting Only the First Row from a Related Table in SQL Joins

Database joins often retrieve multiple matching rows from related tables. This article addresses the scenario where you need to limit the join to a single row per parent record, even if multiple matches exist. This prevents duplicate results in your query output.

The Challenge

Consider two tables: Orders and LineItems. Typically, an order has one line item, but some orders might have multiple. When displaying order details, showing only one line item per order is crucial, otherwise duplicates clutter the results.

Initial Approach (and its failure)

A naive attempt to use TOP 1 within the join directly fails because the inner query can't access the outer table's columns (like OrderID).

The Solution: CROSS APPLY and INNER JOIN

The most effective approach uses CROSS APPLY (available in SQL Server 2005 and later) or a clever INNER JOIN for older versions.

Using CROSS APPLY (SQL Server 2005 and later)

CROSS APPLY generates a rowset for each row in the outer table, enabling a correlated subquery. This subquery then filters and selects a single row from the related table.

<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>

Using INNER JOIN (SQL Server pre-2005)

For older SQL Server versions lacking CROSS APPLY, an INNER JOIN with a subquery achieves the same result:

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

Important Note on Determinism:

The TOP 1 clause is inherently non-deterministic without an ORDER BY clause. To guarantee consistent results (i.e., always selecting the same "first" line item), add an ORDER BY clause within the inner query (e.g., ORDER BY LineItems.SomeColumn). This ensures predictable selection of the line item.

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