MySQL allows us to easily add a primary key constraint to an existing column. We can use the ALTER TABLE statement to achieve this outcome.
We can also use the ADD PRIMARY KEY clause, which can be useful for adding composite primary keys.
Tag: constraints
What is a Composite Primary Key?
Primary keys are a crucial part of relational database management systems (RDBMSs). Most of the time we create a primary key constraint against a single column that is the unique identifier for the table.
But we can also create primary key constraints against more than one column. When we do this, we call it a composite primary key.
Composite keys can be handy when we don’t have a single column that contains unique values, but multiple columns can be combined to create a unique value.
Continue readingCreate a Composite Primary Key in MySQL
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.
How to Skip Rows that Fail a CHECK Constraint in MySQL
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.
2 Ways to List CHECK Constraints in MySQL
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.
How to Change a CHECK Constraint’s Enforcement State in MySQL
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.
2 Ways to Verify Whether a CHECK Constraint is Enforced in MySQL
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.
How to Drop a CHECK Constraint in MySQL
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.
How to Enforce a CHECK Constraint in MySQL
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.
How to Add a CHECK Constraint to an Existing Table 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.