Why Doesn't My SQL `NOT IN` Clause Work with NULL Values?

Troubleshooting SQL's NOT IN Clause and NULL Values

This article addresses a common problem encountered when using the NOT IN clause in SQL queries: unexpectedly empty result sets due to NULL values. The scenario involves selecting records from the Stock table (in the Inventory database) that aren't present in the Products table (in the Subset database).

The NOT IN clause fails to function as expected when the foreignStockId column in the Products table contains NULL values. The presence of even a single NULL in the NOT IN subquery's result set causes the entire query to return no rows.

Solutions:

To correct this, we need to explicitly exclude NULL values. Two effective approaches are presented:

1. Using IS NOT NULL:

This approach filters out NULLs from the NOT IN subquery:

SELECT stock.IdStock, stock.Descr
FROM   [Inventory].[dbo].[Stock] stock
WHERE  stock.IdStock NOT IN (SELECT foreignStockId
                             FROM   [Subset].[dbo].[Products]
                             WHERE  foreignStockId IS NOT NULL);

2. Using NOT EXISTS:

The NOT EXISTS clause provides a more efficient and often clearer alternative:

SELECT stock.idstock, stock.descr
FROM   [Inventory].[dbo].[Stock] stock
WHERE  NOT EXISTS (SELECT *
                   FROM   [Subset].[dbo].[Products] p
                   WHERE  p.foreignstockid = stock.idstock);

Understanding the Problem: Three-Valued Logic

SQL employs three-valued logic: True, False, and Unknown. A WHERE clause must evaluate to True for a row to be included in the result set. The issue arises because 'A' <> NULL evaluates to Unknown. When Unknown is present in a NOT IN comparison, the entire expression becomes Unknown, resulting in an empty result set.

Illustrative Example:

Consider this:

Predicate	True	False	Unknown
'A' NOT IN ('X','Y',NULL)	True	True	Unknown

The Unknown result stems from the comparison of 'A' with NULL. Therefore, the entire WHERE clause evaluates to Unknown, and no rows are returned.

By employing either IS NOT NULL or NOT EXISTS, we bypass this ambiguity and ensure the query returns the intended results. The NOT EXISTS approach is often preferred for its improved performance and readability in such scenarios.

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