Как PK и UNIQUE влияют на NULL?

Question

Accepted Answer

**PRIMARY KEY** — NULL **запрещён**. **UNIQUE** — в SQL Server **один NULL** допустим (multiple NULL treated as distinct in unique index implementation — verify per DBMS).

## Разбор

- ANSI: UNIQUE allows multiple NULLs in some databases.
- PK columns implicitly NOT NULL.
- Filtered unique index для optional unique business keys.

## Итог

PK no NULLs; UNIQUE often one NULL (SQL Server semantics).