datatype_mismatch argument of WHERE must be type boolean, not type … — 42804

Symptoms

The expression in a WHERE (or HAVING/CHECK/ON) clause did not evaluate to boolean. PostgreSQL is strict about boolean context and raises SQLSTATE 42804 (datatype_mismatch).

• Often from WHERE flag = 1 on a non-boolean integer, or WHERE somecolumn that is text.
• The message names the offending type, e.g. integer.
• PostgreSQL does not implicitly treat non-zero numbers as true.

What the server log shows

ERROR:  argument of WHERE must be type boolean, not type integer
LINE 1: SELECT * FROM t WHERE 1;
                              ^

Why PostgreSQL raises this — what the manual says

As Section 9.1 Logical Operators explains:

A WHERE, HAVING, or CHECK condition must evaluate to the boolean type, but the supplied expression has some other type; turn it into a comparison (for example col = value or col IS NOT NULL) or cast it explicitly to boolean.

WHERE filters rows by evaluating a boolean predicate per row. PostgreSQL does not coerce integers or strings to boolean, so a non-boolean expression in that position is a type mismatch and fails with 42804.

Common causes

• Treating an integer/text column as a truthy flag (WHERE active where active is int).
• Forgetting the comparison operator (WHERE status instead of WHERE status = 'open').
• Porting code from a database with truthy semantics.

How to fix it

1. Write an explicit comparison: WHERE active = 1 or WHERE active <> 0.
2. Use a real boolean column/type where you want truthiness.
3. Cast intentionally if needed: WHERE (active)::boolean (note: only 0/1-style ints cast cleanly).

Related & next steps

• SQLSTATE 42804 — Datatype Mismatch (full reference)
• Message: operator does not exist …
• Glossary: Boolean

Reference: PostgreSQL 18 Section 9.1 “Logical Operators”.