Code the classes correctly.
Code the SQL correctly.
Expansion per Ali's request below:
If you are going to make the diagramming tool expressive enough to capture the full range of expressiveness of the the programming language, you have essentially recreated the programming language, but in a non-supported way (what happens when you move to C#5.0?). If you have some of the expressiveness but not all, then the tool will be limited in its ability to generate the code, and you'll have to add hand-coding to make it just right. This means you'll then not be able to make design changes in your diagram tool for fear of knocking out your hand-coded code.
Bonus point: How do you version control your diagrams?