Create the test tables using ON DELETE CASCADEĬonstraint t2_t1_fk foreign key (t1_id) references t1 (id) on delete cascadeĬonstraint t3_t2_fk foreign key (t2_id) references t2 (id) on delete cascade All of the foreign key columns are optional. All the referential integrity constraints are defined using the ON DELETE CASCADE clause. The following code creates three tables in a grandparent-parent-child relationship. The functionality requires all referential integrity constraints involved to be defined using the ON DELETE CASCADE clause. Oracle 12c introduced the CASCADE clause of the TRUNCATE TABLE command, allowing you to recursively truncate down the tables in a hierarchy. ![]() CASCADE in Oracle Database 12c Release 1 (12.1) In this tutorial, you have learned how to use the MySQL ON DELETE CASCADE referential action for a foreign key to delete data automatically from the child tables when you delete data from the parent table.Home » Articles » 12c » Here TRUNCATE TABLE. You can query this data from the referential_constraints in the information_schema database as follows: USE information_schema Ĭonstraint_schema = 'database_name' AND referenced_table_name = 'parent_table' AND delete_rule = 'CASCADE' Code language: SQL (Structured Query Language) ( sql )įor example, to find tables that associated with the buildings table with the CASCADE deletion rule in the classicmodels database, you use the following query: USE information_schema Ĭonstraint_schema = 'classicmodels' AND referenced_table_name = 'buildings' AND delete_rule = 'CASCADE' Code language: SQL (Structured Query Language) ( sql ) Sometimes, it is useful to know which table is affected by the ON DELETE CASCADE referential action when you delete data from a table. Tips to find tables affected by MySQL ON DELETE CASCADE action Some table types do not support foreign keys such as MyISAM so you should choose appropriate storage engines for the tables that you plan to use the MySQL ON DELETE CASCADE referential action. Notice that ON DELETE CASCADE works only with tables with the storage engines that support foreign keys e.g., InnoDB. Query data from rooms table: SELECT * FROM rooms Code language: SQL (Structured Query Language) ( sql )Īs you can see, all the rows that reference to building_no 2 were automatically deleted. We have three rooms that belong to building no 1 and two rooms that belong to the building no 2. Query data from the rooms table: SELECT * FROM rooms Code language: SQL (Structured Query Language) ( sql ) ( 'Showroom', 2) Code language: SQL (Structured Query Language) ( sql ) Insert rows into the rooms table: INSERT INTO rooms(room_name,building_no) Query data from the buildings table: SELECT * FROM buildings Code language: SQL (Structured Query Language) ( sql ) ( 'ACME Sales', '5000 North 1st Street CA 95134') Code language: SQL (Structured Query Language) ( sql ) VALUES( 'ACME Headquaters', '3950 North 1st Street CA 95134'), Insert rows into the buildings table: INSERT INTO buildings(building_name,address) Notice that the ON DELETE CASCADE clause at the end of the foreign key constraint definition. Create the rooms table: CREATE TABLE rooms ( ) Code language: SQL (Structured Query Language) ( sql ) ![]() Create the buildings table: CREATE TABLE buildings (īuilding_no INT PRIMARY KEY AUTO_INCREMENT, The following are steps that demonstrate how the ON DELETE CASCADE referential action works. You also want the rows in the rooms table that refers to building number 2 will be also removed. WHERE building_no = 2 Code language: SQL (Structured Query Language) ( sql ) 2 in the buildings table as the following query: DELETE FROM buildings For example, when you delete a row with building no. When you delete a row from the buildings table, you also want to delete all rows in the rooms table that references to the row in the buildings table. The relationship between the buildings and rooms tables is one-to-many (1:N) as illustrated in the following database diagram: A room would not exist without a building. ![]() However, each room belongs to one only one building. In this database model, each building has one or many rooms. ![]() Suppose that we have two tables: buildings and rooms. Let’s take a look at an example of using MySQL ON DELETE CASCADE. However, MySQL provides a more effective way called ON DELETE CASCADE referential action for a foreign key that allows you to delete data from child tables automatically when you delete the data from the parent table. In the previous tutorial, you learned how to delete data from multiple related tables using a single DELETE statement. Summary: in this tutorial, you will learn how to use MySQL ON DELETE CASCADE referential action for a foreign key to delete data from multiple related tables.
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