Check() Constraint in SQL Server

CHECK() Constraint in SQL server is a data validation technique. Using CHECK() constraints, we can minimise data issues that could cause by bad application code or by incorrect queries by user at the database level.

Consider a table like below:

CREATE TABLE CourseSchedule (

    courseId int NOT NULL,

    course_year int NOT NULL,

    course_start_date date,

    course_end_date date,

    PRIMARY KEY (courseId, course_year) )

Let's say we need users (or applications) to insert year as 4 digit year and year should be value between 2000 and 3000. Then we can define a check constraint like below.

CREATE TABLE CourseSchedule (

    courseId int NOT NULL,

    course_year int NOT NULL,

    course_start_date date,

    course_end_date date,

    PRIMARY KEY (courseId, course_year),

    CHECK(course_year >= 2000 and course_year <= 3000))

You can give a name to the constraint. This is really helpful when debugging issue. So you know which table and which column it happened.

E.g.

CREATE TABLE CourseSchedule (

    courseId int NOT NULL,

    course_year int NOT NULL,

    course_start_date date,

    course_end_date date,

    PRIMARY KEY (courseId, course_year),

    CONSTRAINT my_constraint1 CHECK(course_year >= 2000 and course_year <= 3000))

Check constraint can be declared in the line of the column, if it only applied to that column

E.g.

CREATE TABLE CourseSchedule (

    courseId int NOT NULL,

    course_year int NOT NULL CHECK(course_year >= 2000 AND course_year <= 3000), 

    course_start_date date,

    course_end_date date,

    PRIMARY KEY (courseId, course_year))

But if it reference multiple columns, it has to be on its own line.

E.g.

CREATE TABLE CourseSchedule (

    courseId int NOT NULL,

    course_year int NOT NULL,

    course_start_date date,

    course_end_date date,

    PRIMARY KEY (courseId, course_year),

    CONSTRAINT my_constraint2 CHECK(course_start_date <= course_end_date))

If you want to drop a check constraint:

ALTER TABLE CourseSchedule 

DROP CONSTRAINT my_constraint2