3653
MySQLERRORNotableQueryHIGH confidence
Column count mismatch in SELECT (variant 2)
Production Risk
Medium — Query fails; application may receive unexpected errors.
How to reproduce
trigger — this will error
trigger — this will error
SELECT a, b FROM t1 UNION SELECT c FROM t2;
expected output
ERROR 3653 (HY000): The used SELECT statements have a different number of columns.
Fix
Match column counts in UNION
Match column counts in UNION
SELECT a, b FROM t1 UNION SELECT c, d FROM t2;
Why this works
Ensures both UNION operands have the same number of columns.
What not to do
✕
Version notes
Sources
Official documentation ↗
MySQL 8.0 — 3653 ER_WRONG_NUMBER_OF_COLUMNS_IN_SELECT2
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