NOT IN vs. NOT EXISTS: When Should You Choose NOT EXISTS for Database Queries?

The difference between NOT IN and NOT EXISTS

In the field of database query, we often encounter the problem of whether to use NOT IN or NOT EXISTS. While the execution plans may indicate that their performance is similar, there are some subtle differences to consider when making your choice.

Prefer NOT EXISTS

Generally speaking, many people recommend choosing NOT EXISTS first. This is mainly because it is more robust in handling situations where the columns involved in the query may be null in the future.

Handling of NULL values

NOT IN behaves differently than NOT EXISTS when the column is nullable. NOT IN explicitly excludes NULL values ​​from the results, while NOT EXISTS only excludes rows for which the subquery returns NULL values.

To illustrate this, consider the following query:

<code class="language-sql">SELECT ProductID, ProductName
FROM Products p
WHERE p.ProductID NOT IN (
    SELECT ProductID
    FROM [Order Details]
)</code>

If the ProductID column in the [Order Details] table can contain NULL values, this query will return all products that do not have an associated order or have a ProductID of NULL in the [Order Details] table. On the other hand, the following query using NOT EXISTS will only return products that do not have an associated order:

<code class="language-sql">SELECT ProductID, ProductName
FROM Products p
WHERE NOT EXISTS (
    SELECT 1
    FROM [Order Details] od
    WHERE p.ProductId = od.ProductId
)</code>

Impact of execution plan

While the execution plan may indicate that the two queries perform identically, certain changes in column nullability can change this behavior. If a column becomes nullable and NOT IN is used, the query needs to perform additional checks for NULL values, which may negatively impact performance.

Conclusion

When choosing NOT IN and NOT EXISTS, always consider the potential nullability of the columns involved in the query. NOT EXISTS provides greater robustness and clarity in handling NULL values, making it preferred in most scenarios.

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