What are different types of constraints? describe in brief?

Answer 1

UNIQUE - Requires a unique value, no duplicate records
NOT NULL - Requires a value, can't be blank
PRIMARY KEY - Designates a table's Primary Key

Answer 2

I agree:
UNIQUE - Requires a unique unduplicated record
NOT NULL - Requires a value, can't be left empty, forces an entry to be present
PRIMARY KEY - Only accepts a unique entry in the field where this is present