12_Primary_Key_vs_Unique_Key.md

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!