php - Finding username of the last post in mysql table joining 3 tables -

i know doing qry wrong - taking on 10secs results come w/ 3000 rows of data... have 3 tables:
users
id
username

data: 1|tom 2|dick 3|harry 

posts
id
id_users

data: 1|1 2|1 3|1 4|2 5|2 6|3 

cronjobs
id
id_post
id_wall

data:   id|id_post|id_wall 1|1|1 2|1|2 3|1|3 4|1|4 5|1|5 6|2|5 7|4|3 8|6|3 9|4|4 

a user make post post put on 1 or more walls , stored in cronjobs table. id on cronjobs auto increment. need username , last post on each wall. in above example

dick last person post on wall 4 post 4
harry last person post on wall 3 post 6
tom last person post on wall 5 post 2
tom last person post on wall 2 post 1
tom last person post on wall 1 post 1

here qry im using, know using in clause select inside it, killing this....

select    c.id,   c.id_post,   c.id_wall id_wall,   p.id_users user_id,   u.name username    cronjobs c,   posts p,   users u  c.id in    (select max(id)        cronjobs    group id_wall)    , c.id_post = p.id    , p.id_users = u.id  order c.id  

any appreciated!

try this

select    c.id,   c.id_post,   c.id_wall id_wall,   p.id_users user_id,   u.name username    cronjobs c,   posts p,   users u ,   (select max(id) id        cronjobs    group id_wall) table_id   c.id_post = p.id    , p.id_users = u.id  , table_id.id  =c.id order c.id