UNION: queries must have the same number of columns

Every SELECT combined with UNION, INTERSECT, or EXCEPT has to return the same number of columns, in a compatible order. Here's the fix and the type-compatibility trap that comes right after it.

• SQLite: SELECTs to the left and right of UNION do not have the same number of result columns
• PostgreSQL: ERROR: each UNION query must have the same number of columns
• MySQL: The used SELECT statements have a different number of columns
• SQL Server: All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists.

Loading editor…

The cause

UNION (and INTERSECT, EXCEPT) stacks the results of two SELECTs into one result set — which only makes sense if both sides have the same shape: same number of columns, in an order where each pair is a compatible type. The left side selects two columns here; the right side selects one.

The fix — match the column count and order

Loading editor…

Columns are matched by position, not by name — the result's column headers come from the first SELECT regardless of what the second one calls them. If you need a placeholder to make the counts match, select a literal:

Loading editor…

The next error waiting after this one: type mismatch

Once the column counts agree, engines also expect each paired column to be a compatible type. Stack a text column against a numeric one and some engines error, others silently coerce — which is its own trap, since a query that "works" may just be quietly converting everything to text or truncating a number. Keep types aligned on purpose:

Loading editor…

UNION vs UNION ALL

UNION also de-duplicates the combined result, which means a hidden sort or hash step on every query — often unwanted, and wasteful if the two sides can't produce duplicates anyway. If you don't need de-duplication, UNION ALL skips it and is the better default:

Loading editor…