What is the difference between on delete set null and on delete cascade? 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 the difference between delete and delete cascade in SQL?
In most cases, there is no difference between the two options. The difference is visible when the delete operation is triggered by some other operation, such as delete cascade from a different table, delete via a view with a UNION, a trigger, etc. Let’s take a look at an example.
What is the difference between restrict and delete Cascade?
Again, see comments. ON UPDATE CASCADE ON DELETE CASCADE means that if you UPDATE OR DELETE the parent, the change is cascaded to the child. This is the equivalent of AND ing the outcomes of first two statements. RESTRICT means that any attempt to delete and/or update the parent will fail throwing an error.
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 does onDon delete Cascade mean?
ON DELETE CASCADE means that if the parent record is deleted, any child records are also deleted. This is not a good idea in my opinion. You should keep track of all data that's ever been in a database, although this can be done using TRIGGERs. (However, see caveat in comments below).
What is delete set null and delete cascade in SQL?
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. It is used in conjunction with ON DELETE or ON UPDATE.
What is on delete cascade and on delete Set Default?
ON DELETE CASCADE : if a row of the referenced table is deleted, then all matching rows in the referencing table are deleted. ON DELETE SET NULL : if a row of the referenced table is deleted, then all referencing columns in all matching rows of the referencing table to be set to null.
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 does delete on Cascade mean?
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.
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.
Should I 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 Cascade delete in MySQL?
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 does on update cascade mean?
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.
What is add on delete set NULL in MySQL?
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. If you specify a SET NULL action, make sure that you have not declared the columns in the child table as NOT NULL .
What is add on delete cascade in SQL Server?
Edit table and columns specification by clicking … as shown in the below image.Select the parent table and the primary key column in the parent table. ... In the INSERT and UPDATE specifications, select Cascade for the delete rule.Click on Close and save the table in the designer.More items...•
Can foreign key be null?
Foreign keys allow key values that are all NULL , even if there are no matching PRIMARY or UNIQUE keys.
What is on delete restrict?
ON DELETE RESTRICT means you can't delete a given parent row if a child row exists that references the value for that parent row. If the parent row has no referencing child rows, then you can delete that parent row.
Best Answer
A foreign key with a cascade delete means that if a record in the parent table is deleted, then the corresponding records in the child table with automatically be deleted. This is called a cascade delete. Cascade constrains Deletes all foreign keys that reference the table to be dropped, then drops the table.
Answers
A foreign key with a cascade delete means that if a record in the parent table is deleted, then the corresponding records in the child table with automatically be deleted. This is called a cascade delete. Cascade constrains Deletes all foreign keys that reference the table to be dropped, then drops the table.
In Some Databases There Is No Difference at All
In Oracle, there is no RESTRICT keyword. The only option is NO ACTION. In MySQL, there is no difference between ON DELETE RESTRICT and ON DELETE NO ACTION.
If There Is a Difference Though..
The databases IBM DB2, PostgreSQL (for INITIALLY DEFERRED constraints), HSQLDB, and SQLite know the difference between ON DELETE RESTRICT and ON DELETE NO ACTION.
Example: ON DELETE RESTRICT
Let’s take a look at an example. There are four tables here: r0, r1, r2, r3. Table r0 is the parent table for tables r1 and r2 with the DELETE CASCADE constraint, the table r2 is the parent of r3 with the DELETE CASCADE constraint, and r1 is the parent of r3 with the ON DELETE RESTRICT constraint.
Example: ON DELETE NO ACTION
Now take a look at this example. The only difference is that now the constraint on the reference between table n1 and n3 is ON DELETE NO ACTION (plus I have renamed the r* tables to n* tables). The data in the tables is the same as above.
