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If you want to change a foreign key without dropping it you can do: ALTER TABLE child_table_name WITH CHECK ADD FOREIGN KEY(child_column_name) REFERENCES parent_table_name (parent_column_name) ON DELETE CASCADE
usually my default is: ON DELETE RESTRICT ON UPDATE CASCADE. with some ON DELETE CASCADE for track tables (logs--not all logs--, things like that) and ON DELETE SET NULL when the master table is a 'simple attribute' for the table containing the foreign key, like a JOB table for the USER table.
20 cze 2013 · There are five options for ON DELETE and ON UPDATE that can apply to the FOREIGN KEY. These are called <referential actions>, directly from the SQL:2011 spec. ON DELETE CASCADE: if a row of the referenced table is deleted, then all matching rows in the referencing table are deleted.
SET NULL: Delete or update the row from the parent table and set the foreign key column or columns in the child table to NULL. Both ON DELETE SET NULL and ON UPDATE SET NULL clauses are supported.
MySQL supports ON UPDATE and ON. DELETE foreign key references in CREATE TABLE and ALTER TABLE statements. The available referential actions are RESTRICT, CASCADE, SET NULL, and NO ACTION (the default). SET DEFAULT is also supported by the MySQL Server but is currently rejected as invalid by InnoDB.
17 sie 2017 · ON DELETE SET NULL and ON UPDATE SET NULL in MySQL. Like CASCADE, we can use SET NULL on delete and update operations. The foreign key column from the child table (books) will be set to NULL when the parent record gets updated or deleted.
The reference_option determines the action that MySQL will take when values in the parent key columns are deleted (ON DELETE) or updated (ON UPDATE). MySQL has five reference options: CASCADE , SET NULL , NO ACTION , RESTRICT , and SET DEFAULT .
