IN vs EXISTS

Both IN and EXISTS are used in subqueries to filter rows, but they behave differently and are not always interchangeable.

Understanding their differences helps you write SQL that is both efficient and accurate.

At first glance, they seem to do the same thing:

Return rows from one table that match values from another.

Behind the scenes, they process data differently — which impacts performance, accuracy, and scalability.

• IN works like a lookup: it collects all subquery results first, then compares them.
• EXISTS checks row by row whether any condition is satisfied, and stops at the first match.

This difference becomes important when:

• Subqueries are large or correlated
• NULLs are present in the result
• You care about query speed

While both may give the same result, they have trade-offs:

• IN can fail unexpectedly with NULLs
• EXISTS is often faster in correlated subqueries
• Many SQL engines optimize EXISTS more effectively under heavy load

Use IN when:

• The subquery is small and returns a fixed list
• You want a clean value comparison

Use EXISTS when:

• You’re filtering against another table with many rows
• The subquery is correlated to the outer query
• You want to avoid NULL-related issues

Check out the slide deck on the right to see visual comparisons and practical examples of IN vs EXISTS in action.