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How does MySQL handle foreign key constraints?
MySQL allows us to define a foreign key constraint on the child table. MySQL defines the foreign key in two ways: Using CREATE TABLE Statement. Using ALTER TABLE Statement….We have defined the foreign key in the contact table as:
- FOREIGN KEY (Customer_Id) REFERENCES customer(ID)
- ON DELETE CASCADE.
- ON UPDATE CASCADE.
Can enum be a primary key?
Using an ENUM for a primary key means that adding a new key would involve modifying the table since the ENUM has to be modified before you can insert a new key.
Is enum a basic data type?
Enumerations. An enumeration is a data type that consists of a set of named values that represent integral constants, known as enumeration constants. An enumeration is also referred to as an enumerated type because you must list (enumerate) each of the values in creating a name for each of them.
How is a foreign key composed in MariaDB?
It is composed by a column (or a set of columns) in a table called the child table, which references to a column (or a set of columns) in a table called the parent table. If foreign keys are used, MariaDB performs some checks to enforce that some integrity rules are always enforced.
What are the different types of constraint in MariaDB?
There are four types of table constraints: Sets the column for referencing rows. Values must be unique and not null. Sets the column to reference the primary key on another table. Requires values in column or columns only occur once in the table. Checks whether the data meets the given condition.
Can a column be an index in MariaDB?
The columns in the child table must be an index, or the leftmost part of an index. Index prefixes are not supported (thus, TEXT and BLOB columns cannot be used as foreign keys). If MariaDB automatically creates an index for the foreign key (because it does not exist and is not explicitly created), its name will be index_name.
When to use on update clause in MariaDB?
When a row in the parent table is deleted and at least one child row exists, MariaDB performs an action which depends on the ON DELETE clause of the foreign key. When a value in the column referenced by a foreign key changes and at least one child row exists, MariaDB performs an action which depends on the ON UPDATE clause of the foreign key.