how to use check in sql code example

Example 1: sql constraint check value in list

ALTER TABLE <table>
ADD CONSTRAINT chk_val CHECK (col in ('yes','no','maybe'))

Example 2: sql check

Adds a constraint that limits the value which can be added to a column.
Example 1 (MySQL): Makes sure any users added to the users table are 18
or over.
CREATE TABLE users (
first_name varchar(255),
age int,
CHECK (age>=18)
);
Example 2 (MySQL): Adds a check after the table has already been
created.
ALTER TABLE users
ADD CHECK (age>=18);