Apache DerbyBigQueryDb2 (LUW)H2MariaDBMySQLOracle DBPostgreSQLSQL ServerSQLite200520072009201120132015201720192021⊘ 3.5.7 - 3.40.0⊘ 2008R2 - 2022✓ 15+⊘ 8.3 - 14⊘ 11gR1 - 21c⊘ 5.0 - 8.0.32⊘ 5.1 - 10.10⊘ 1.4.191 - 2.1.214⊘ -
  1. Doesn’t support null-able columns in unique constraints (T591)

Unique constrains accept an optional nulls not distinct modifier to specify that one null value rules out other null values in the same column of the constraint.


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

The optional nulls not distinct specification is in a draft for the upcoming SQL standard as optional feature F292, “UNIQUE null treatment”.

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.

About the Author

Photo of Markus Winand

Markus Winand is the SQL Renaissance Ambassador. He is on a mission to introduce developers to the evolution of SQL in the 21st century. Markus can be hired as trainer, speaker and consultant via

Buy his Book

Cover of “SQL Performance Explained”: Squirrel running on grass

The essence of SQL tuning in 200 pages

Buy now!
(paperback and/or PDF)

Paperback also available at

Hire Markus

Markus offers SQL training and consulting for developers working at companies of any size.
Learn more »


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

Connect with Markus Winand

Markus Winand on LinkedInMarkus Winand on XINGMarkus Winand on Twitter
“modern SQL” by Markus Winand is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Unported License.
Legal | Contact | NO WARRANTY | Trademarks | Privacy and GDPR | CC-BY-NC-ND 3.0 license