Which is best way in maintaining large business logic whether it should in windows programm or database program

See more:

SQL

server

C#5.0

Express

I have Created business logic in Database which is as bellows
1. There are lots students data in tables
2. Bases on their rank in respective courses I fetch students (Student near by 1 Lakh)
3. Loop through preferences and find in Steps find whether candidates fits any seats.Then Allocates the candidates in respective seat if possible.
4. This procedure when starts executing then it takes time more than 5 hours as it is likely to have recursive logic.
5. Now I want to have this logic should be maintained in C# Code where we can handle logic as well as when changes from client can easily managed .But in database we have to modify Procedure every time.
6. Also in C# Windows its not working as takes more than 5 hours.
7. Also when if any logic is failed then for tracing (Debugging) is very difficult as we are using SQL Server Express 2012 .
8. For this we want to move logic in C# Code where we can easily manage Business Logic easily.

What I have tried:

I made simple queries with indexes properly build after watching query execution plan
Even then procedure takes more time .I tried to debug procedure but failed as it hangs or gives run time exception .

Also i tried to maintain log table and print statement but even then its very difficult to find out issue as Store procedure have more steps and complex logic .
For this I want to move all logic in Coding so that we can debug and maintain logic as well .Also we can modify logic if there is change in business rule.