php/mysql delete oldest entries

[acctman]

Hi I need help with a query that will cycle through the 'com_for' (userid) field and then only keep the latest 200 comments and delete everything after. Example: So if 'com_for' userid 39 has 250 comments then delete the oldest 50 comments. The com_time field holds the time in this format 1133173361

 

table: rate_picture_comments

fields: com_id , com_for , com_comment , com_time 

 

This has been tried with sql only, (I posted in the Mysql section) but mysql wasn't able to handle multiple selects and deleting kept getting a unknown table error and after some researching online it was suggestion in a case like this its best to off load the process to a scripting language like php.