Primary key vs Unique key

Primary key

• The only key in a table

• Used to uniquely identify each record in the table

  • For this role, it has Unique & Not Null properties

• Can consist of one row or multiple rows within a table

• When composed of multiple rows, it's called a Composite Primary Key

• Primary Key automatically creates an Index, which helps to quickly search and manage records

Unique key

• Plays the role of not allowing duplicates (Unique)

  • Can identify each record

• Allows Null

  • This means there can be multiple Null values as long as the row is not duplicated

• Multiple Unique Keys can be defined within one table

• If a Primary Key is defined, that row automatically becomes a Unique Key

  • but, the reverse is not true

    • Unique Key cannot replace the role of Primary Key!