[PHP-DB] Delete Subquery

[Brock Jimmy D Contr 74 MDSS/SGSI]

I've been able to figure how to use a delete subquery to delete all rows, but how do I 
use a delete subquery to delete only one row?

 

I have two tables: users and roles with the following fields:

 

Users:

userid

roleid

 

Roles:

roleid

 

I want to be able to delete a rowid from the roles table, but only if the roleid does 
not exist in the users table.

 

thanks