Beginning learning SQL with C#/ASP.NET

Question

Sorry if this has been asked elsewhere, but I couldn't find a clear answer anywhere.

I have decided to begin learning to use relational databases a bit more, namely SQL. This is a major beginners question but its probably essential to get started on.

I'm basically a little confused the best practice on how to utilize SQL (or other). At college i have accessed databases (using JSON strings) for things such as mobile apps, but i have never actually designed and built a database myself, as my tutor made the mentioned database for us to access himself.

Lets say I have a C# application that holds genealogy information (i.e. families and their members) and i wanted to store each individual on a database. Would I, simply use the structure I already have but save to fields in a database instead of an xml or text document? Or does it work the other way, i.e. do I create a database with required fields then just retrieve this from the database in a c# application and manipulate the data as I so wish, so the application would be entirely different (so the c# application basically doesn't really hold/store any data and just works on whats fed from the database)?

Whats troubling me is that usually where I would store my c# objects in a dictionary or list for example, would i instead just retrieve straight from the database? or retrieve from the and store the data into a normal structure and work from there (surely this would defeat the point of fast-searching from a database)?

I may be over-thinking it slightly. Hope that makes sense. Thanks in advance

Answer 1

Would I, simply use the structure I already...

or

do I create a database with required fields...

I think that is the crux of your question.

Starting from the database

For me, when building an application that uses a backend database, an Entity-Relationship diagram is pretty crucial. I found quite a nice little tutorial for you here: http://www.sum-it.nl/cursus/dbdesign/english/index.php3 but you can easily find one that suits your learning style. The key point is that you are trying to model the problem domain (the real world out there that needs your application) in a way that your application can somehow capture. Once you have an E-R diagram of related tables, it is easier to figure out the details. Using SQL Management Studio for SQL Server 2008 (Express edition) you can create a few basic tables and build the E-R diagram right there and have it generate relationships for you. You can then, at your leisure, examine the SQL used to achieve that and refine accordingly.

Personally, I always start by examining the problem domain, then I build the E-R diagram, then I build the database. I start building the C# application when I'm reasonably confident the database reflects the problem domain.

Starting from your C# application

However, what really matters is that you model the real world in a meaningful and effective way. In your case you already have a starting point in structures you've created in C# and you can use them to give you a starting point to build the E-R diagram. If you find it easier to get a C# application going and then build a database that reflects it, that should be fine. Perhaps you already have an approach that helps you capture the problem domain effectively. It's an iterative process whatever you do: building the C# code might reveal problems with the underlying database design and vice versa.

Diagramming - E-R or UML?

I'm personally convinced that this whole business is so complicated that you really need some diagrams.

• to visualise your database, use an E-R diagram
• to visualise your C# application use a UML class diagram

As you head towards a working application, you'll see how these 2 diagrams begin to match or at least reflect eash other pretty closely. In both cases, (entities or classes) understanding the relationship between objects will be really important when you query the database because it is crucial to understand relationships between tables (especially using 1-to-many relationships to resolve a complex many-to-many relationship) and various techniques for joining tables in queries (INNER or OUTER joins etc) No matter how clever your C# application is, you will at some point need to understand at least some of the complexities of the SQL language - and it is easier if you can refer to an E-R diagram.

Where to store?

Whats troubling me is that usually where I would store my c# objects in a dictionary or list for example, would i instead just retrieve straight from the database?

In the database, without a doubt. A C# class called Family would have a property FamilyName, say, with a setter method built in. If you discover a spelling mistake and want to change the name, the setter method would open a connection to the database, run an UPDATE query with the specified family name, (and probably the family id) as a parameter, and update the underlying field accordingly. Retrieving data would involve running a SELECT query etc.

Conclusion

Do some tutorials on how to examine a problem domain, create an entity-relationship diagram and build a set of related tables based on the diagram. I'm convinced that way you'll find it much easier to keep track of the C# classes that you build to communicate with the backend database.

Here's an example of a simple E-R diagram for families and their members:

To begin with you might think members and family could be in one table, but then you discover that creates a lot of duplication so you separate that out into family and member table with a one-to-many relationship, but then you realise that, through marriage for instance, people can belong to more than one family and you need to create a many-to-many relationship. I think the E-R diagram is the best place to work out that kind of complexity.