Generic way to identify field in table

[Drazalon]

I am writing a sql editor (sqlite3).I can display a table and allow users to edit any value in the table but I now need some way of identifying the value editted. The problem is that I don't know what the primary key is and I don't think it's that good an idea to say "update table set a=b where x=123 and y=123 and z=123 and..." for all the fields that aren't "a".I'm using jquery (and ajax) and php.Any ideas?