Primary Key vs Unique Key
Primary Key vs Unique Key
Both constraints enforce column uniqueness, but serve distinct roles in relational schema architecture.
Comparison Table
| Attribute | Primary Key | Unique Key |
|---|---|---|
| Quantity per Table | Exactly ONE | Multiple allowed |
| Nullability | NOT NULL (strictly forbidden) | Allows NULLs (1 or multiple) |
| Default Index | Clustered Index (InnoDB/MSSQL) | Non-Clustered Index |
| Semantic Purpose | Main entity identifier | Secondary unique attributes |
| Foreign Key Target | Primary target for FKs | Valid target for FKs |