UNIQUE
accepts multiple
null
BigQuery
Db2 (LUW)
MariaDB
MySQL
Oracle DB
PostgreSQL
SQL Server
SQLite
2005
2007
2009
2011
2013
2015
2017
2019
2021
✓ 3.5.7+
⊘ 2008R2 - 2022
c
✓ 8.3+
⚠ 11gR1 - 21c
a
✓ 5.0+
✓ 5.1+
Deviates for multi-column constrains (see above)
Cannot create UNIQUE constraints on possibly null columns
Accepts only one
null
occurrence
Doesn’t support UNIQUE constraints