CHECK

CHECK constraint is a condition defined to a column or a table to limit the value that can be inserted into it. 

For instance, in our students table, let us assume that we had another column "Age". The minimum age requirement for this class is 16. So to ensure that the age of a student is atleast 16, we define CHECK constraint in the table students as follow:

CREATE TABLE students (

student_id   INT  NOT NULL,

first_name   VARCHAR(255)   NOT NULL,

last_name   VARCHAR(255)   NOT NULL,

email   varchar(255),

age   INT   NOT NULL,

CHECK (Age >= 16)

);

What if the table was already created? 

ALTER TABLE students 

ADD CHECK (Age >= 16);