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Delete parent if it's not referenced by any other child

I have an example situation: parent table has a column named id, referenced in child table as a foreign key.

When deleting a child row, how to delete the parent as well if it's not referenced by any other child?

like image 991
zeroDivisible Avatar asked Apr 04 '13 11:04

zeroDivisible


People also ask

How do I delete parent as well as child table?

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 SQL Server.

Can we delete a row from parent table which is associated to child table as foreign key relationship?

The point of a foreign key constraint is to prevent orphan records in the child table. So, no, it's not possible to do that, unless you drop the foreign key relationship. If you rely on 'ON DELETE CASCADE', then deleting the parent record will result in all the corresponding children to be deleted.

Which clause allows the deletion of child records if the parent records are deleted?

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.

Can you delete foreign key from child table?

It lives within the child's table space. FOREIGN KEY constraints are available for row deletion ( ON DELETE ) and row updates ( ON UPDATE ).


2 Answers

In PostgreSQL 9.1 or later you can do this with a single statement using a data-modifying CTE. This is generally less error prone. It minimizes the time frame between the two DELETEs in which a race conditions could lead to surprising results with concurrent operations:

WITH del_child AS (
    DELETE FROM child
    WHERE  child_id = 1
    RETURNING parent_id, child_id
    )
DELETE FROM parent p
USING  del_child x
WHERE  p.parent_id = x.parent_id
AND    NOT EXISTS (
   SELECT 1
   FROM   child c
   WHERE  c.parent_id = x.parent_id
   AND    c.child_id <> x.child_id   -- !
   );

db<>fiddle here
Old sqlfiddle

The child is deleted in any case. I quote the manual:

Data-modifying statements in WITH are executed exactly once, and always to completion, independently of whether the primary query reads all (or indeed any) of their output. Notice that this is different from the rule for SELECT in WITH: as stated in the previous section, execution of a SELECT is carried only as far as the primary query demands its output.

The parent is only deleted if it has no other children.
Note the last condition. Contrary to what one might expect, this is necessary, since:

The sub-statements in WITH are executed concurrently with each other and with the main query. Therefore, when using data-modifying statements in WITH, the order in which the specified updates actually happen is unpredictable. All the statements are executed with the same snapshot (see Chapter 13), so they cannot "see" each others' effects on the target tables.

Bold emphasis mine.
I used the column name parent_id in place of the non-descriptive id.

Eliminate race condition

To eliminate possible race conditions I mentioned above completely, lock the parent row first. Of course, all similar operations must follow the same procedure to make it work.

WITH lock_parent AS (
   SELECT p.parent_id, c.child_id
   FROM   child  c
   JOIN   parent p ON p.parent_id = c.parent_id
   WHERE  c.child_id = 12              -- provide child_id here once
   FOR    NO KEY UPDATE                -- locks parent row.
   )
 , del_child AS (
   DELETE FROM child c
   USING  lock_parent l
   WHERE  c.child_id = l.child_id
   )
DELETE FROM parent p
USING  lock_parent l
WHERE  p.parent_id = l.parent_id
AND    NOT EXISTS (
   SELECT 1
   FROM   child c
   WHERE  c.parent_id = l.parent_id
   AND    c.child_id <> l.child_id   -- !
   );

This way only one transaction at a time can lock the same parent. So it cannot happen that multiple transactions delete children of the same parent, still see other children and spare the parent, while all of the children are gone afterwards. (Updates on non-key columns are still allowed with FOR NO KEY UPDATE.)

If such cases never occur or you can live with it (hardly ever) happening - the first query is cheaper. Else, this is the secure path.

FOR NO KEY UPDATE was introduced with Postgres 9.4. Details in the manual. In older versions use the stronger lock FOR UPDATE instead.

like image 144
Erwin Brandstetter Avatar answered Sep 21 '22 17:09

Erwin Brandstetter


delete from child
where parent_id = 1

After deleted in the child do it in the parent:

delete from parent
where
    id = 1
    and not exists (
        select 1 from child where parent_id = 1
    )

The not exists condition will make sure it will only be deleted if it does not exist in the child. You can wrap both delete commands in a transaction:

begin;
first_delete;
second_delete;
commit;
like image 22
Clodoaldo Neto Avatar answered Sep 21 '22 17:09

Clodoaldo Neto