# Supporting uniqueness constraints

A column that has a uniqueness constraint must have a unique value for every record.

Primary key columns automatically require uniqueness. Uniqueness can also be required for other columns. For example, in a `users` table, `userid` is the primary key column, but `username` also must be unique.

The following generators can be used with columns that have uniqueness constraints:

* [Alphanumeric String Key](https://docs.tonic.ai/app/generation/generators/generator-reference/alphanumeric-string-key)
* [ASCII Key](https://docs.tonic.ai/app/generation/generators/generator-reference/ascii-key)
* [Character Substitution](https://docs.tonic.ai/app/generation/generators/generator-reference/character-substitution)
* [Conditional](https://docs.tonic.ai/app/generation/generators/generator-reference/conditional)
* [Finnish Personal Identity Code](https://docs.tonic.ai/app/generation/generators/generator-reference/finnish-personal-identity-code)
* [FNR](https://docs.tonic.ai/app/generation/generators/generator-reference/fnr)
* [Geo](https://docs.tonic.ai/app/generation/generators/generator-reference/geo)
* [IBAN](https://docs.tonic.ai/app/generation/generators/generator-reference/iban)
* [Integer Key](https://docs.tonic.ai/app/generation/generators/generator-reference/integer-key)
* [Null](https://docs.tonic.ai/app/generation/generators/generator-reference/null)
* [Numeric String Key](https://docs.tonic.ai/app/generation/generators/generator-reference/numeric-string-key)
* [Passthrough](https://docs.tonic.ai/app/generation/generators/generator-reference/passthrough)
* [Random UUID](https://docs.tonic.ai/app/generation/generators/generator-reference/random-uuid)
* [Regex Mask](https://docs.tonic.ai/app/generation/generators/generator-reference/regex-mask)
* [Sequential Integer](https://docs.tonic.ai/app/generation/generators/generator-reference/sequential-integer)
* [SIN](https://docs.tonic.ai/app/generation/generators/generator-reference/sin)
* [Unique Email](https://docs.tonic.ai/app/generation/generators/generator-reference/unique-email)
* [URL](https://docs.tonic.ai/app/generation/generators/generator-reference/url)
* [UUID Key](https://docs.tonic.ai/app/generation/generators/generator-reference/uuid-key)