Unique key constraint

UNIQUE constraint to enforce uniqueness of a column
the column shouldn't allow any duplicate values
To create the unique key using a query:
Alter Table Table_Name
Add Constraint Constraint_Name Unique(Column_Name)

Both primary key and unique key are used to enforce, the uniqueness of a column. So, when do you choose one over the other?
A table can have, only one primary key. If you want to enforce uniqueness on 2 or more columns, then we use unique key constraint.

What is the difference between Primary key constraint and Unique key constraint? This question is asked very frequently in interviews.
1. A table can have only one primary key, but more than one unique key
2. Primary key does not allow nulls, where as unique key allows one null

To drop the constraint
1. Right click the constraint and delete.
Or
2. Using a query
Alter Table tblPerson
Drop COnstraint UQ_tblPerson_Email

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