Delete with join on the same table and limit clause - mysql

Question

I have a MySql table like this,
Session_Id Subscriber_Id Status
-------------------------------
abc 1234 Started
bcd 1235 Started
bcd 1235 Finished
And I need to delete rows with status 'Started' only if followed by a 'Finished'. This is a huge table, so I would be deleting 1000 records at a time.
I read up a few similar threads,
Multiple-table DELETE LIMIT,
DELETE FROM `table` AS `alias` ... WHERE `alias`.`column` ... why syntax error?
and tried the below queries,
mysql> delete a.* from FSESSION as a, FSESSION as b where a.status='Started' and b.status='Finished' and a.session_id=b.session_id limit 1000;
ERROR 1064 (42000): 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 'limit 1000' at line 1 ----> If I remove 'limit' this works
mysql> DELETE FROM v USING `FSESSION` AS v WHERE status = 'Started' and exists(select 0 from FSESSION t where t.status='Finished' and v.session_id=t.session_id) limit 1000;
ERROR 1064 (42000): 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 'limit 1000' at line 1 ---->
Does'nt work even when limit is removed.
I use MySql version 5.1.56-ndb-7.1.15. Please suggest a way to do this. Thanks!

Answer 1

UPDATED based on your comments
DELETE f
FROM fsession f JOIN
(
SELECT session_id, subscriber_id
FROM fsession
WHERE status IN('Started', 'Finished')
GROUP BY Session_Id, Subscriber_Id
HAVING MAX(status = 'Started') > 0
AND MAX(status = 'Finished') > 0
LIMIT 1000 -- limit here means how many pairs of rows, so it's effectively 2x rows
) q ON f.session_id = q.session_id
AND f.subscriber_id = q.subscriber_id
To know how many rows were deleted use ROW_COUNT()
SELECT ROW_COUNT();
Here is SQLFiddle demo

Answer 2

Try these queries:
CREATE TEMPORARY TABLE FSESSION_TEMP
(
Temp_Session_Id varchar(255)
);
INSERT INTO FSESSION_TEMP
(
SELECT a.Session_Id FROM
FSESSION a,
FSESSION b
WHERE
a.Status = 'Started' and
b.Status = 'Finished' and
a.Session_ID = b.Session_ID
LIMIT 2
);
DELETE FROM
FSESSION
WHERE
Session_ID in (SELECT * FROM FSESSION_TEMP)
;
DROP TABLE FSESSION_TEMP;
Here is the SQL Fiddle. I've set LIMIT at 2 to show that it works. You still need to change it to 1000.
It seems that you need to use a temporary table for this for 2 reasons:
1) MySQL doesn't let you delete from a table when you use the same table in a subquery
2) For the multiple-table syntax, DELETE deletes from each tbl_name the rows that satisfy the conditions. In this case, ORDER BY and LIMIT cannot be used (Taken straight out of the MYSQL DELETE manual)
There is one way to circumvent the restrictions by using the DELETE syntax with JOIN and joining the original table (where rows will be deleted from) with a derived table (identical to the above temporary one):
DELETE fdel
FROM
FSESSION fdel
JOIN
(
SELECT a.Session_Id
FROM
FSESSION a
JOIN
FSESSION b
ON a.Session_ID = b.Session_ID
WHERE
a.Status = 'Started' and
b.Status = 'Finished'
LIMIT 2
) ftemp
ON ftemp.Session_ID = fdel.Session_ID
;