T591, “UNIQUE constraints of possibly null columns”

Related

• Implementation-defined

  • ID106 - The unique null treatment when no explicit unique null treatment is specified

  • IA201 - Whether two rows whose column values are all the null value are considered unique with nulls distinct, or unique with nulls not distinct

• Tutorials:

  • NULL-Aware Comparison: is [not] distinct from

  • NULL: Indicating the Absence of Data

  • The Three-Valued Logic of SQL

Normative References

The optional feature T591, “UNIQUE constraints of possibly null columns” is defined in ISO/IEC 9075-2:2023. It first appeared in the release of 1999. In SQL-92 it was part of the intermediate level.