Constraints are used to set a rule to allow or add restrictions on database tables in MYSQL.  That decides what values should be stored in columns.

• Constraints are used to identify unique records in the database table.
• Constraints help to set limit the value which should be entered into the column.

Constraints in MYSQL

• Primary Key constraint
• Foreign Key constraint
• Unique Key constraint
• check constraints

Primary key Constraint in MYSQL

It is used to define, each record in a table, and its primary condition is that it should have a unique value.
Example:

CREATE TABLE roles(
role_id INT AUTO_INCREMENT,
role_name VARCHAR(50),
PRIMARY KEY(role_id)
);

Foreign key Constraint in MYSQL

Foreign key is used in the table which points the primary key to another table.
Example:

CREATE TABLE products(
prd_id int not null auto_increment primary key,
prd_name varchar(355) not null,
prd_price decimal,
cat_id int not null,
FOREIGN KEY fk_cat(cat_id)
REFERENCES categories(cat_id)
ON UPDATE CASCADE
ON DELETE RESTRICT
)ENGINE=InnoDB;

Unique key Constraint in MYSQL

It is used when you want to identify unique records in the database table.
Example:

ALTER TABLE CUSTOMERS
ADD CONSTRAINT myUniqueConstraint UNIQUE(AGE, SALARY);

Check Constraint in MYSQL

When you want to limit the value which should be entered into the column, MySQL check constraints can be used.
Example:

ALTER TABLE CUSTOMERS
MODIFY AGE INT NOT NULL CHECK (AGE >= 18 );