SQL Server UNIQUE constraints allow you to ensure that the data stored in a column, or a group of columns, is unique among the rows in a table
CREATE TABLE user( id INT IDENTITY PRIMARY KEY, name VARCHAR(255) NOT NULL, email VARCHAR(255), UNIQUE(email) ); If you enter a duplicate email id in the database, the error will show. Violation of UNIQUE KEY constraint 'UQ__user__AB23436417240E4E'. Cannot insert duplicate key in object 'user'. The duplicate key value is (test@gmail.com)