Scalar subquery must return one row

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Accepted Answer

A scalar subquery is expected to produce a single value. If it returns more than one row, most databases raise 'subquery returned more than one row' at runtime. If it returns zero rows, it produces NULL (not an error). This is a common bug: SELECT name, (SELECT salary FROM employees WHERE dept = e.dept) fails when a dept has multiple employees. The fix: add an aggregate like MAX/MIN, add a LIMIT 1, or restructure as a JOIN.