Deleting multiple rows from multiple tables - mysql

Question

I have three tables:
`MEMBERS`
with
`NAME` varchar(24) UNIQUE KEY
`LAST_LOGGED_IN` int(11) - It is a timestamp!
`HOMES`
with
`OWNER` varchar(24)
`CARS`
with
`OWNER` varchar(24)
I use InnoDB for these tables, now my actual question is: How do I remove rows within all the tables if the UNIX_TIMESTAMP()-MEMBERS.LAST_LOGGED_IN > 864000?
I'm trying to remove inactive members' rows, and this is the hardest thing yet. I have about 40K rows, and increasing. I clean it regularly with DELETE FROM MEMBERS WHERE UNIX_TIMESTAMP()-LAST_LOGGED_IN> 864000
Any of your help would be extremely grateful! Thanks!!

Answer 1

If you have already removed rows from the MEMBERS table, and you want to remove the rows from the other two tables where the value of the OWNER column does not match a NAME value from any row in the MEMBERS table:
DELETE h.*
FROM `HOMES` h
LEFT
JOIN `MEMBERS` m
ON m.`NAME` = h.`OWNER`
WHERE m.`NAME` IS NULL
DELETE c.*
FROM `CARS` c
LEFT
JOIN `MEMBERS` m
ON m.`NAME` = c.`OWNER`
WHERE m.`NAME` IS NULL
(N.B. these statements will also remove rows from the HOMES and CARS tables the OWNER column as a NULL value.)
I strongly recommend you to run a test of these statements using a SELECT before you run the DELETE. (Replace the keyword DELETE with the keyword SELECT, i.e.
-- DELETE h.*
SELECT h.*
FROM `HOMES` h
LEFT
JOIN `MEMBERS` m
ON m.`NAME` = h.`OWNER`
WHERE m.`NAME` IS NULL
Going forward, if you want to keep these tables "in sync", you may consider defining FOREIGN KEY constraints with the ON CASCADE DELETE option.
Or, you can use a DELETE statement that removes rows from all three tables:
DELETE m.*, h.*, c.*
FROM `MEMBERS` m
LEFT
JOIN `HOMES` h
ON h.`OWNER` = m.`NAME`
LEFT
JOIN `CARS` c
ON c.`OWNER` = m.`NAME`
WHERE UNIX_TIMESTAMP()-m.`LAST_LOGGED_IN` > 864000
(N.B. the predicate there cannot make use of an index on the LAST_LOGGED_IN column. An equivalent predicate with a reference to the "bare" column will be able to use an index.
WHERE m.`LAST_LOGGED_IN` < UNIX_TIMESTAMP()-864000
or an equivalent:
WHERE m.`LAST_LOGGED_IN` < UNIX_TIMESTAMP(NOW() - INTERVAL 10 DAY)
For best performance, you would need indexes on both HOMES and CARS with a leading column of OWNER, e.g.
... ON `HOMES` (`OWNER`)
... ON `CARS` (`OWNER`)

Answer 2

I don't use InnoDB so I had to look it up, but it does appear to support Referential Integrity. If you set relationships and then turn on ON DELETE CASCADE, the database itself will enforce the rules... i.e., when you delete a Member, the DBMS will take care of deleting the associated Homes and Cars.
See here and here, they might help.