F292, “UNIQUE null treatment”


BigQuery 2025-09-02aaDb2 (LUW) 12.1.2aaDerby 10.17.1.0H2 2.3.232MariaDB 12.0.2MySQL 9.3.0Oracle DB 23.9PostgreSQL 17SQL Server 2022SQLite 3.50.0unique 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 (ID106). 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 (IA201)0

BigQuery 2025-09-02aaaaDb2 (LUW) 12.1.2aaaaDerby 10.17.1.0H2 2.3.232MariaDB 12.0.2MySQL 9.3.0Oracle DB 23.9PostgreSQL 17SQL Server 2022SQLite 3.50.0ID106: nulls distinctID106: nulls not distinctIA201: nulls distinctIA201: nulls not distinct
  1. Doesn’t support null-able columns in unique constraints (T591)

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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Footnotes

  1. ISO/IEC 9075-2:2023 §4.25.3.2, in particular NOTE 52.

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