Difference between primary key and unique key
database, mysql, primary-key, sql, unique-key
Solution
Primary Key:
- There can only be one primary key constraint in a table
- In some DBMS it cannot be `NULL` - e.g. MySQL adds `NOT NULL`
- Primary Key is a unique key identifier of the record
Unique Key:
- Can be more than one unique key in one table
- Unique key can have `NULL` values
- It can be a candidate key
- Unique key can be `NULL` ; multiple rows can have `NULL` values and therefore may not be considered "unique"
Problem
I'm using a MySQL database. In which situations should I create a unique key or a primary key?