MySQL CHECK Constraint

 MySQL CHECK Constraint

Summary: in this tutorial, you will learn how to use MySQL CHECK constraint to ensure that values stored in a column or group of columns satisfy a Boolean expression.

MySQL 8.0.16 implemented the SQL check constraint. If you use MySQL with the earlier versions, you can emulate a CHECK constraint using a view WITH CHECK OPTION or a trigger.

Introduction to the MySQL CHECK constraint

Prior to MySQL 8.0.16, the CREATE TABLE allows you to include a table CHECK constraint. However, the CHECK the constraint is just parsed and ignored:

CHECK(expression)

As of MySQL 8.0.16, the CREATE TABLE supported essential features of table and column CHECK constraints for all storage engines.

Here is the syntax:

[CONSTRAINT [constraint_name]] CHECK (expression) [[NOT] ENFORCED]

In this syntax:

First, specify the name for the check constraint that you want to create. If you omit the constraint name, MySQL automatically generates a name with the following convention:

table_name_chk_n

where n is an ordinal number 1,2,3… For example, the names of CHECK constraints for the parts table will be parts_chk_1, parts_chk_2, …

Second, specify a Boolean expression which must evaluate to TRUE or UNKNOWN for each row of the table. If the expression evaluates to FALSE, the values violate the constraint or a constraint violation occurs.

Third, optionally specify enforcement clause to indicate whether the check constraint is enforced:

• Use ENFORCED or just omit the ENFORCED clause to create and enforce the constraint.
• Use NOT ENFORCED to create the constraint but do not enforce it.

As mentioned earlier, you can specify a CHECK constraint as a table constraint or column constraint.

A table CHECK a constraint can reference multiple columns while the column CHECK a constraint can refer to the only column where it is defined.

MySQL CHECK constraint examples

Let’s take some examples of using the CHECK constraints.

1) MySQL CHECK constraint – column constraint example

This statement creates a new parts table:

CREATE TABLE parts (
    part_no VARCHAR(18) PRIMARY KEY,
    description VARCHAR(40),
    cost DECIMAL(10,2 ) NOT NULL CHECK (cost >= 0),
    price DECIMAL(10,2) NOT NULL CHECK (price >= 0)
);

In this statement, we have two column CHECK constraints: one for the cost column and the other for the price column.

Because we did not explicitly specify the names for the CHECK constraints, MySQL automatically generated names for them.

To view the table definition with the CHECK constraint name, you use the SHOW CREATE TABLE statement:

SHOW CREATE TABLE parts;

Here is the output:

As you can see clearly from the output, MySQL generated the check constraint parts_chk_1 and parts_chk_2.

Once the CHECK constraints are in place, whenever you insert or update a value that causes the Boolean expression to evaluate to be false, MySQL rejects the change and issues an error.

This statement inserts a new row into the parts table:

INSERT INTO parts(part_no, description,cost,price) 
VALUES('A-001','Cooler',0,-100);

MySQL issued an error:

Error Code: 3819. Check constraint 'parts_chk_2' is violated.

Because the value of the price the column is negative which causes the expression price > 0 evaluates to FALSE that results in a constraint violation.

2) MySQL CHECK constraint – table constraint example

First, drop the parts table:

DROP TABLE IF EXISTS parts;

Then, create a new parts table with one more table CHECK constraint:

CREATE TABLE parts (
    part_no VARCHAR(18) PRIMARY KEY,
    description VARCHAR(40),
    cost DECIMAL(10,2 ) NOT NULL CHECK (cost >= 0),
    price DECIMAL(10,2) NOT NULL CHECK (price >= 0),
    CONSTRAINT parts_chk_price_gt_cost 
        CHECK(price >= cost)
);

The following new clause defines a table CHECK the constraint that ensures the price is always greater than or equal to cost:

CONSTRAINT parts_chk_price_gt_cost CHECK(price >= cost)

Because we explicitly specify the name for the CHECK constraint, MySQL just creates the new constraint with the specified name.

Here is the definition of the parts table:

SHOW CREATE TABLE parts;

The table CHECK constraint appears at the end of the table definition after the column list.

This statement attempts to insert a new part whose price is less than cost:

INSERT INTO parts(part_no, description,cost,price) 
VALUES('A-001','Cooler',200,100);

Here is the error due to the constraint violation:

Error Code: 3819. Check constraint 'parts_chk_price_gt_cost' is violated.

In this tutorial, you have learned about the MySQL CHECK constraints to ensure values stored in a column satisfy a Boolean condition.