EXISTS stops at first match (efficient). Index foreign keys in subquery. EXISTS often faster than IN for large datasets. Compare with JOIN using EXPLAIN.

EXISTS stops at first match (efficient)

Index columns in subquery WHERE clause

EXISTS often faster than IN for large datasets

Use EXISTS instead of COUNT(*) > 0

Test EXISTS vs JOIN with EXPLAIN

Using JOIN when EXISTS is sufficient

Not indexing subquery columns

Using IN instead of EXISTS with NULLs

Using COUNT(*) > 0 instead of EXISTS

Not testing performance differences