SQL Interview Prep · Lesson

Column Count and Type Compatibility

The rules result sets must satisfy to be combined and common mismatch errors.

The Compatibility Rules Interviewers Probe

Every set operation (UNION, INTERSECT, EXCEPT) requires the two result sets to be union-compatible. Interviewers ask this to see if you know the silent ways these queries break.

Two rules govern compatibility: matching column count and compatible data types position by position. Get either wrong and you get an error or a surprising result.

Rule 1: Same Number of Columns

Both queries must project the same number of columns. If the first query returns three columns and the second returns two, the database rejects the statement.

This is the single most common set-operation error in interviews, especially after someone edits one branch and forgets the other.

SELECT id, name, city FROM a
UNION
SELECT id, name FROM b;
-- ERROR: each UNION query must have the same number of columns

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