Cardinality Violation — SQLSTATE 21000

SQLSTATE 21000 condition cardinality_violation class 21 — Cardinality Violation severity ERROR
Reproduced & verified on PostgreSQL 14.23, 15.18, 16.14, 17.10 and 18.4 — identical message on every version.
Last reviewed 29 May 2025 · Reproduced live with the SQL on this page.

! Symptoms Free

A scalar context received more than one row.

  • The error is written to the server log and returned to the client carrying SQLSTATE 21000.
  • Any driver (libpq, JDBC, psycopg, npgsql, pgx) surfaces this code in its error object so you can branch on it programmatically.
  • PL/pgSQL can trap it by name: EXCEPTION WHEN cardinality_violation THEN.

1 Environment & reproduce Free

Severity: ERROR  |  PostgreSQL versions: 12, 13, 14, 15, 16, 17

Reproduce with the exact statement and read the full message in the server log (raise log_min_messages / set log_min_error_statement for more context).

? Root cause Free

A subquery used where a single value is required returned multiple rows, or a multi-row update target was ambiguous.

Common causes:

  • A scalar subquery returning more than one row.
  • SELECT INTO matching multiple rows.
  • An UPDATE ... FROM join producing several source matches per target row.

3 Recovery & verify Free

Steps to resolve 21000:

  1. Add LIMIT 1 with a deterministic ORDER BY, or aggregate the subquery.
  2. Make join keys unique so each target row matches at most one source row.
  3. In PL/pgSQL use SELECT ... INTO STRICT only when exactly one row is guaranteed.

Reference: PostgreSQL error codes — Class 21 (Cardinality Violation).