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.
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.
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 readingA 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.