Difference between Primary & Secondary Keys

  PRIMARY KEY: 
 

• Cannot contain Null values or duplicate rows.
• Cannot be updated.
• Can be defined as a foreign key  key of in other table.
• only one primary key can be defined for one table.

 FOREIGN KEY:

• Can contain NULL values or rows.
• Can be updated .
• which must be primary key of another table.
• multiple foreign keys can be defined for one table.