What is the purpose of the RESTRICT, CASCADE, SET NULL, and NO ACTION options?


When I'm going to create a foreign key type relationship between two tables in MySQL, I can specify some additional options in the ON UPDATE and ON DELETE events that are associated with the change and deleting records.

The options are:

  • A practical illustration example using the NO ACTION option, see below:

    CREATE TABLE 'usuariorelsupermercado' (
        'idUsuario' INT(11) NOT NULL,
        'idSupermercado' INT(11) NOT NULL,
        INDEX 'fk_usuario_rel' ('idUsuario'),
        INDEX 'fk_supermercado_rel' ('idSupermercado'),
        CONSTRAINT 'fk_supermercado_rel' FOREIGN KEY ('idSupermercado') REFERENCES 'supermercado' ('idSupermercado') ON UPDATE NO ACTION ON DELETE NO ACTION,
        CONSTRAINT 'fk_usuario_rel' FOREIGN KEY ('idUsuario') REFERENCES 'usuario' ('idUsuario') ON UPDATE NO ACTION ON DELETE NO ACTION

    I have some questions about these options.


    • What is the purpose of each of these options?
    • What do these options influence in my database?
    asked by anonymous 09.06.2017 / 19:16

    1 answer


    They are options for foreign keys, trying to simplify as much as possible:

    RESTRICT: Refuses to update or delete a record from the parent table if there are records in the child table.

    CASCADE: Updates or deletes records from the child table automatically when updating or deleting a record from the parent table.

    SET NULL: Set to null the value of the field in the child table when updating or deleting the parent table record.

    NO ACTION: Equivalent to RESTRICT.

    There is still the SET DEFAULT: Sets the value of the column in the child table, as the value set to default by deleting or updating a record in the parent table.

    More information: link

    Examples: You have a City and Client table, Assuming:

     1|São Paulo
    id|Nome  |Cidade_id

    When updating / deleting the registration of the city of são paulo:

    RESTRICT / NO ACTION: The bank will reject the command, returning a foreign key violation exception.

    CASCADE: If you change the value of the City's id column, the value of the City_id column in the Customer table will also be changed. If you exclude the city of São Paulo, you will also be excluded Client 1, So-and-so.

    SET NULL: The value of the Id_Id column of records that are using the value 1, São Paulo, will be set to null.

    Complementing: What do these options influence in my database?

    Maintains data integrity.

    09.06.2017 / 19:33