CASCADE. It is used in conjunction with ON DELETE or ON UPDATE. It means that the child data is either deleted or updated when the parent data is deleted or updated. SET NULL.
What is the function of on delete cascade on delete cascade?
1 Answer. The explanation is: It is used to preserve referential integrity in the relation. When an attribute of a relation is the foreign key in another relation, deleting it causes referential integrity problems. The on delete cascade solves this problem by forcing us to delete the foreign key first.
What is on delete cascade and on update cascade?
1) ON DELETE CASCADE means if the parent record is deleted, then any referencing child records are also deleted. ON UPDATE defaults to RESTRICT, which means the UPDATE on the parent record will fail.
What is on delete cascade and on delete set null in SQL?
ON DELETE CASCADE : SQL Server deletes the rows in the child table that is corresponding to the row deleted from the parent table. ON DELETE SET NULL : SQL Server sets the rows in the child table to NULL if the corresponding rows in the parent table are deleted.
What is on update cascade in MySQL?
The ON UPDATE CASCADE tells the database that when an update occurs on the referenced column from the parent table (“ id ”), it must automatically update the matching rows in the child table (“ books ”) with the new value.
Why do we use on delete cascade in SQL?
Use the ON DELETE CASCADE option to specify whether you want rows deleted in a child table when corresponding rows are deleted in the parent table. If you do not specify cascading deletes, the default behavior of the database server prevents you from deleting data in a table if other tables reference it.
What is on delete cascade in SQL explain clearly with example?
ON DELETE CASCADE constraint is used in MySQL to delete the rows from the child table automatically, when the rows from the parent table are deleted. For example when a student registers in an online learning platform, then all the details of the student are recorded with their unique number/id.
What is the functions of on Deletecascade?
Explanation: It is used to preserve referential integrity in the relation. When an attribute of a relation is the foreign key in another relation, deleting it causes referential integrity problems. The on delete cascade solves this problem by forcing us to delete the foreign key first.
What is on delete cascade in Oracle?
A foreign key with cascade delete means that if a record in the parent table is deleted, then the corresponding records in the child table will automatically be deleted. This is called a cascade delete in Oracle.
What is on delete Set NULL?
A foreign key with “set null on delete” means that if a record in the parent table is deleted, then the corresponding records in the child table will have the foreign key fields set to NULL. The records in the child table will not be deleted in SQL Server.
Is on delete No action default?
The default is NO ACTION. So, you can elide ON DELETE NO ACTION if you like and it will work just the same. NO ACTION means that nothing will happen when you delete from your Subject table to the Topic table.
Can a primary key be NULL?
A primary key defines the set of columns that uniquely identifies rows in a table. When you create a primary key constraint, none of the columns included in the primary key can have NULL constraints; that is, they must not permit NULL values.
What is the difference between Cascade and restrict?
The CASCADE option directs the DBMS Server to revoke the specified privileges plus all privileges and objects that depend on the privileges being revoked. The RESTRICT option directs the DBMS Server not to revoke the specified privilege if there are any dependent privileges or objects.
What is difference between Cascade delete and restrict delete in data integrity?
CASCADE :will propagate the change when the parent changes. (If you delete a row, rows in constrained tables that reference that row will also be deleted, etc.) RESTRICT :causes the attempted DELETE of a parent row to fail.
What is Cascade constraints in SQL?
CASCADE CONSTRAINTS. Specify CASCADE CONSTRAINTS to drop all referential integrity constraints that refer to primary and unique keys in the dropped table. If you omit this clause, and such referential integrity constraints exist, then the database returns an error and does not drop the table.
