To create a check constraint in SQL, first create the column on which the constraint will apply. Before the closing parenthesis of the table definition, use the following formula:

CONSTRAINT name CHECK (expression) 

The CONSTRAINT and the CHECK keywords are required. As an object, make sure you provide a name for it. Inside the parentheses that follow the CHECK operator, enter the expression that will be applied. Here is an example that will make sure that the hourly salary specified for an employee is greater than 12.50:

CREATE TABLE Employees
(
	[Employee Number] nchar(7),
	[Full Name] varchar(80),
	[Hourly Salary] smallmoney,
	CONSTRAINT CK_HourlySalary CHECK ([Hourly Salary] > 12.50)
);

It is important to understand that a check constraint it neither an expression nor a function. A check constraint contains an expression and may contain a function as part of its definition.