Why does this UNION break at runtime?

Question

Accepted Answer

UNION, UNION ALL, INTERSECT, and EXCEPT all require the same number of columns in both queries. The first query returns 2 columns (id, name); the second returns 1 column (name) - this is a mismatch and causes a syntax/runtime error. Column names do not need to match - the first query's column names are used for the result. Data types must be compatible though.