what is the check constraint in sql code example

Example 1: check constraint in sql

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    City varchar(255),
    CONSTRAINT CHK_Person CHECK (Age>=18 AND City='Sandnes')
);

Example 2: check constraint in sql

#Add Check
ALTER TABLE Persons
ADD CONSTRAINT CHK_PersonAge CHECK (Age>=18 AND City='Sandnes');

#Drop Check 
ALTER TABLE Persons
DROP CHECK CHK_PersonAge;