UNIQUE accepts multiple null

BigQueryDb2 (LUW)MariaDBMySQLOracle DBPostgreSQLSQL ServerSQLite200520072009201120132015201720192021✓ 3.5.7+⊘ 2008R2 - 2019d✓ 8.3+⚠ 11gR1 - 21cc✓ 5.0+✓ 5.1+
  1. Doesn’t support UNIQUE constraints
  2. Cannot create UNIQUE constraints on possibly null columns
  3. Deviates for multi-column constrains (see above)
  4. Accepts only one null occurrence