Can I use… F292, “UNIQUE null treatment”

Apache DerbyBigQueryaaDb2 (LUW)aaH2MariaDBMySQLOracle DBPostgreSQLSQL ServerSQLiteunique nulls distinctunique nulls not distinct
  1. Doesn’t support null-able columns in unique constraints (T591)

Implementation Defined Behavior

The default null treatment of unique constraints is implementation defined. Note that nulls not distinct unique constraints may still treat null values as distinct from each other for rows where all constraint columns are null.0

Apache DerbyBigQueryaaDb2 (LUW)aaH2MariaDBMySQLOracle DBbcPostgreSQLSQL ServerSQLitedefault: nulls distinctdefault: nulls not distinct
  1. Doesn’t support null-able columns in unique constraints (T591)
  2. If all constraint columns are null
  3. If some but not all constraint columns are null

Normative References

F292, “UNIQUE null treatment” is an optional feature in ISO/IEC 9075-2:2023. If first appeared in ISO/IEC 9075-2:2023.

Note that SQL:2016 specifies the behaviour of nulls distinct as the only way to cope with null values in unique constraints. In the mentioned draft the default behaviour is implementation defined.

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  1. CD 9075-2:202x(E), Unique constraints (in particular NOTE 52).

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