A composite primary key is a primary key that consists of two or more columns. Together they will always provide a unique value within the table. In other words, the combination of both values will be unique across all rows – no two rows will share the same combined value.

In MySQL we can create a composite primary key with the PRIMARY KEY clause of the CREATE TABLE statement. We can also use the ADD PRIMARY KEY statement to add a composite primary key to an existing table that doesn’t already have a primary key.

Normally if we try to insert data that violates a CHECK constraint in MySQL, we get an error, and the whole INSERT operation fails (including any conforming rows).

But it’s possible to change this, so that any conforming rows are inserted, and only the non-conforming rows are skipped.

We can do this by using the IGNORE clause.

Ever since the release of MySQL 8.0.16, we’ve had the ability to create CHECK constraints in MySQL. At some point, we may want to return a list of CHECK constraints that have been created in a database or against a given table.

Fortunately, we can use the information_schema.check_constraints view to do just that.

We can alternatively use the information_schema.table_constraints view to get the same info.

When we create or alter a CHECK constraint in MySQL, we have the option of setting it to ENFORCED or NOT ENFORCED. This determines whether or not MySQL will enforce the rules of the constraint whenever data that violates the constraint tries to enter the database.

We can use the ALTER TABLE statement to change the enforcement state of an existing CHECK constraint in MySQL, as shown in the following example.

When we create a CHECK constraint in MySQL, we have the option of defining it as ENFORCED or NOT ENFORCED.

MySQL CHECK constraints are enforced by default, so unless we explicitly define the constraint as NOT ENFORCED, it will automatically be enforced.

In any case, we can use either of the following two methods to check whether or not an existing CHECK constraint is enforced.

If we want to remove a CHECK constraint from a table, but we don’t want to drop the table or column, we can use the ALTER TABLE statement with DROP CHECK.

Once the constraint has been dropped, data can be entered into the table without having to conform to the rules of the CHECK constraint.

If we have a CHECK constraint that’s currently not enforced, we can easily change this so that it’s enforced.

We can use the following code to enforce a CHECK constraint in MySQL.

As of MySQL 8.0.16, we can create CHECK constraints in MySQL. We can create them at the time of creating the table, or we can add the constraint later.

To add a CHECK constraint to an existing table, we can use the ALTER TABLE statement.