You have to design your code from the first to be layered. All data is in the database, next level is accessing / manipulating data - code written in C# using SQL commands. Next layer is C# functionality. This code needs to be very generic / flexible for future additions. etc etc
The major problem with SQL modding is that it is TOO easy to mod, every gamer wants to write their own mods, and do, ending up with a Billion and one mods that don't like each other, crash each other, and turns into a huge mess. I write my own SQL scripts for my own game (CIV 5) but don't publish them. But there are some excellent mods in CIV, professionally done, that i do use.