Unique constraint in MySql

1. Add a username field to instructor table

• ALTER TABLE instructor ADD COLUMN username VARCHAR(20); 

• Note that all usernames are set to null initially
2. Add a unique constraint to the table with respect to usernames

• ALTER TABLE instructor ADD CONSTRAINT unique_username 
  UNIQUE (username)

• Tells MySql that usernames must be unique. Unlike primary keys, null values are allowed.

3. Make some updates

• UPDATE instructor SET username = "srini" WHERE name = "Srinivasan"; #ok

• UPDATE instructor SET username = "srini" WHERE name = "Wu"; #NOT OK