An article on implementing row level security in MySQL has been published
Jun 22, 2009
"Can I have a single database, with only one set of tables to store all clients' information and let each client access only his/her data?" Such questions are not infrequent in MySQL forums. The article introduces a solution of the problem. This solution is based only on built-in MySQL features; moreover, it works independently of application used to connect to the database as all business logic is centralized in the database itself, not in the client apps, and also has no security threats.
See the full article here.