In MySQL, the UNIQUE constraint is used to uniquely identify each record in a table.

UNIQUE Constraint on CREATE TABLE

The following statement creates a UNIQUE constraint on the "id" column when the "employee" table is created: Example

CREATE TABLE emplyee
(
id int NOT NULL,
first_name varchar(45) NOT NULL,
last_name varchar(45),
country varchar(45),
UNIQUE (id)
);

We can also define UNIQUE constraint on multiple column, for this use the below statement:

CREATE TABLE emplyee
(
id int NOT NULL,
first_name varchar(45) NOT NULL,
last_name varchar(45),
country varchar(45),
CONSTRAINT un_employeeId UNIQUE (id,first_name)
);

UNIQUE Constraint on ALTER TABLE

We can also create UNIQUE constraint on a column when the table is already created, for that we use ALTER command.

Syntax:

ALTER TABLE table_name
ADD UNIQUE column_name

Example:

ALTER TABLE employee
ADD UNIQUE (id);

Similarly we can define UNIQUE constraint on multiple columns when the table is already created as below:

ALTER TABLE employee
ADD CONSTRAINT un_employeeId UNIQUE (id,first_name);

To DROP a UNIQUE Constraint

We can also drop a UNIQUE constraint. To drop a UNIQUE constraint, use the below statement:

ALTER TABLE employee
DROP INDEX un_employeeId;

Hope this will help you :)