Constraints in mysql

What is constraint
SQL constraints are used to specify rules for data in a table.

The following are the constraints.

NOT NULL - Ensures that a column cannot have a NULL value.

UNIQUE - Ensures that all values in a column are different.

PRIMARY KEY - The PRIMARY KEY constraint uniquely identifies each record in a database table, (or) A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table.

FOREIGN KEY - A FOREIGN KEY is a key used to link two tables together. Uniquely identifies a row/record in another table.

CHECK - The CHECK constraint is used to limit the value range that can be placed in a column. Ensures that all values in a column satisfies a specific condition.
Example:
CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CHECK (Age>=18)
);

DEFAULT - Sets a default value for a column when no value is specified.

INDEX - Used to create and retrieve data from the database very quickly.