lovesh lovesh - 1 month ago 11
MySQL Question

what is the real cause of mysql error 1442?

well i have looked for a lot of places on the internet for the cause of the

mysql error #1442
which says


Can't update table 'unlucky_table' in stored function/trigger because
it is already used by statement which invoked this stored
function/trigger


some say that this is a bug in mysql or a feature that it doesnt provide.

MySQL triggers can't manipulate the table they are assigned to. All other major DBMS support this feature so hopefully MySQL will add this support soon.

Some claim that this is due to recursive behavior
when you insert a record mysql is doing some lock stuff. you can't insert/update/delete rows of the same table where you insert.. because then the trigger would called again and again.. ending up in a recursion

During the insert/update you have access to the NEW object which contains all of the fields in the table involved. If you do a before insert/update and edit the field(s) that you want to change in the new object it will become a part of the calling statement and not be executed as a separately (eliminating the recursion)

now i cant understand why this is recursive. i have a case in which i have 2 tables
table1
and
table2
and i run an sql query as

update table1 set avail = 0 where id in (select id from table2 where duration < now() - interval 2 hour);


now i have an
after update trigger
on
table1
as

CREATE TRIGGER trig_table1 AFTER UPDATE ON table1
FOR EACH ROW begin
if old.avail=1 and new.avail=0 then
delete from table2 where id=new.id;
end if;


now when i execute the update query i get a 1442 error.
whats recursive in this case?

is this error a lack of feature in mysql?
OR
does this have to do with how mysql executes queries?
OR
is there something logically wrong with executing such queries?

Answer

You cannot refer to a table when updating it.

/* my sql does not support this */
UPDATE tableName WHERE 1 = (SELECT 1 FROM tableName)

From MySQL Docs:

A trigger can access both old and new data in its own table. A trigger can also affect other tables, but it is not permitted to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger. (Before MySQL 5.0.10, a trigger cannot modify other tables.)