Delete, Update with derived tables?

Question

I have just studied FROM clause and derived tables in mysql and most of the websites provided the examples using SELECT command

Example SELECT * FROM (SELECT * FROM usrs) as u WHERE u.name = 'john'

But when I have tried using delete or update command it does not seem to work.

Example DELETE FROM (SELECT * FROM usrs) as u WHERE u.name = 'john'

1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near (SELECT * FROM usrs) as u WHERE u.name = 'john' at line

UPDATE (SELECT * FROM usrs) as u SET u.lname ='smith' WHERE u.name = 'john'

1288 The target table e of the UPDATE is not updatable

So derived tables does not work with delete or update commands? or is there a way to make it work.

Instead of writing the table name for update and delete I want to write a subquery that gets the records and perform the delete operation on that records? Is that possible in mysql?

UPDATED I have to delete a record and i have three tables, the record may exist in any of the table

My approach delete from first table rows effected? quit: else check second table rows effected? quit : else check third table

But if I use UNION ALL I can do this way

Delete from (select * from tb1 union all select * from tb2 union all select * from tb3) e as e.uname = 'john'

but this query does not seem to work , now could anyone tell me how do i delete or update a record when i have more than one table to search. Any help is greatly appreciated.

Answer 1

You can't directly delete from the subquery, but you can still use it if you'd like, you'll just need to use it in a JOIN:

DELETE usrs
FROM usrs
    INNER JOIN (
        SELECT * FROM usrs WHERE name = 'john'
    ) t ON usrs.Id = t.Id

Or you could use IN:

DELETE usrs
WHERE ID IN (
   SELECT ID
   FROM usrs
   WHERE name = 'John'
)

With this said, for this example, I don't know why you'd want a subquery:

DELETE usrs WHERE name = 'John'

---

Edit base on comments. To delete from multiple tables at the same time, you can either have multiple DELETE statements, or you can use something like the following:

delete t1, t2, t3
from (select 'john' as usr) t
  left join t1 on t.usr=t1.usr
  left join t2 on t.usr=t2.usr
  left join t3 on t.usr=t3.usr

• SQL Fiddle Demo

Answer 2

Derived tables exist only for the duration of the parent query they're a member of. Assuming that this syntax and the operations were allowed by MySQL, consider what happens:

a) Your main query starts executing
b) the sub-query executes and returns its results as a temporary table
c) the parent update changes that temporary table
d) the parent query finishes
e) temporary tables are cleaned up and deleted

Essentially you'll have done nothing except waste a bunch of cpu cycles and disk bandwidth.

UPDATE queries DO allow you to join against other tables to use in the WHERE clause, e.g..

UPDATE maintable
LEFT JOIN othertable ON maintable.pk = othertable.fk
SET maintable.somefield='foo'
WHERE othertable.otherfield = 'bar'