The topic describes the DF116 T-SQL code analysis rule.
EXECUTION RULES
Non-scalar subquery is used instead of a scalar subquery.
Using a non-scalar result in a context that anticipates a scalar result can result in errors.
Non-scalar subqueries return multiple rows, which can cause performance degradation, especially if they are used in contexts where a single value is expected. The database engine must process and evaluate all rows returned by the subquery, potentially resulting in unnecessary overhead.