What is the difference between Cascade delete and set NULL on delete?
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 delete set NULL on update cascade?
It means that the child data is either deleted or updated when the parent data is deleted or updated. SET NULL. It is used in conjunction with ON DELETE or ON UPDATE. It means that the child data is set to NULL when the parent data is deleted or updated.
What is 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 does on delete set NULL mean?
What is a foreign key with “Set NULL on Delete” in Oracle? 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.
What is difference between Cascade and set NULL?
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.
What is on delete cascade and on delete Set default?
It means that the child data is set to NULL when the parent data is deleted or updated. SET DEFAULT. It is used in conjunction with ON DELETE or ON UPDATE. It means that the child data is set to their default values when the parent data is deleted or updated.
Should we use on delete cascade?
ON DELETE CASCADE is fine, but only when the dependent rows are really a logical extension of the row being deleted. For example, it’s OK for DELETE ORDERS to delete the associated ORDER_LINES because clearly you want to delete this order, which consists of a header and some lines.
What is on delete cascade and on delete restrict?
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. 2) ON DELETE action defaults to RESTRICT, which means the DELETE on the parent record will fail.
What is on delete set null in SQL?
On Delete Set Null clause sets all the records of the column which is defined as a foreign key in the child table to Null if the corresponding record in the parent table is deleted.
Why do we use Cascade?
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 NULL NULL in SQL?
In SQL null is not equal ( = ) to anything—not even to another null . According to the three-valued logic of SQL, the result of null = null is not true but unknown. SQL has the is [not] null predicate to test if a particular value is null .
Is primary key and unique key same?
Both keys provide a guaranteed uniqueness for a column or a set of columns in a table or relation. The main difference among them is that the primary key identifies each record in the table, and the unique key prevents duplicate entries in a column except for a NULL value.
What happens when you delete a row in SQL?
Set NULL : Sets the column value to NULL when you delete the parent table row. CASCADE : 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.
What is the difference between Cascade and restrict in SQL?
CASCADE : 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 : RESTRICT causes you can not delete a given parent row if a child row exists that references the value for that parent row.
What is the difference between ‘nullify’ and ‘Cascade’?
:nullify will set to null (i.e. if you delete a user, any revisions associated with that user will have their created_by_id set to null) :cascade will cascade the deletion (i.e. if you delete a user, any revisions associated with that user will be deleted too)
What is the difference between SET NULL and restrict in SQL?
(If you delete a row, rows in constrained tables that reference that row will also be deleted, etc.) SET NULL sets the column value to NULL when a parent row goes away. RESTRICT causes the attempted DELETE of a parent row to fail. EDIT: You didn’t ask about them, but the SQL standard defines two other actions: SET DEFAULT and NO ACTION.