What is the Reason large sites don't use MySQL with ASP.NET?

1. Less compatibility issues when you single source.
2. MS SQL Server is the "default" database for ASP.NET apps (see LINQ to SQL, ADO.NET, ApplicationServices etc)
3. Immature .NET tools for other databases. For example, you don't have to worry about a feature or functionality not being supported if you stick with MS SQL Server, other databases might not have full support (e.g. DbLInq, etc.).
4. MS SQL Server is also free to get started, (SQL Server Express) and once you're ready to go public, it's hard to change the Data layer.

I'm in the process of writing an ASP.NET MVC2 site with MySQL as the backend (mainly due to licensing costs) I've implemented DbLinq, but it also means writing a custom Membership/Role provider, and general tweaking of the data layer. It's definitely doable, but it's not as simple as sticking with MS SQL Server. I'm also hoping to move the site over to Mono 2.7 (once it's released) running on a Linux server to sidestep the server licensing issues as well.

The real reason is people usually go with MS SQL Server as .NET comes from the same brand. For instance PHP people always prefer MySQL over other databases. It's all mind set and people don't want to take any risk.

Adding into what AJ said... Remember Facebook also pays C programmers to hack up MySQL code and also PHP code to get things to really work "well" for the amount of traffic they get.

Facebook already made statements in the past and this year about having wished they made a better choice.

As a matter of fact, for coding they're now compiling their PHP down to C++ code using HipHopPHP and about 90% of their servers are running the C++ binaries instead of the PHP scripts.

Their MySQL database might save them a dime or two, but the costs to maintain it, scale it, etc. is extremely intense.

A product like Oracle however would really allow you to scale seamlessly compared to MySQL.

I have a site right now that uses a lot of bandwidth on my database, large number of queries, and the truth is, scaling is a pain in the neck with MySQL and their Clustering product isn't that great and requires a license. Oracle right now has the best "grid" database setup but the costs are insane there...

Also, I code C# as well.. Let me tell you it's MUCH easier to integrate enterprise level sites with SQL Server compared to MySQL.

You use what you know...

(IMHO) The Microsoft tool stack is brilliant. It works well, we learn with it and grow with it, as the technology grows. It becomes easier to use as you become accustomed to it (its quirks and idiosyncrasies).

MySQL is also a brilliant tool. It works, and works well. We could all have religious wars as to what tool is best, but remember it is just a tool to get a job done.

Now let's factor in the cost of the software - Plenty of Fish 2 years ago made $7M, do you really think they care how much their database/server software costs? SO is on BizSpark $0 cost for 3 years (that's got to hurt).

For the sceptics, FaceBook runs MySQL on/for 30K servers and [MySQL Enterprise Unlimited][1] Licences cost $40k so this is not necessarily cheap either.

I don't know about you, but for me when I make a ton of cash, I really won't care how much it "costs", because I am making more with it, than without it!

[1]:

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Any large enterprise site isn't going to care about licensing costs that much. What they want is fast, reliable data access and access to company technical support. They also want something that can easily be partioned to scale and that is designed for huge databases. They also want the easy availablity of performance tuning specialists, datawarehousing and Business Intelligence specialists, database developers, and database administrators. SQL Server and Oracle both meet these criteria. I really don't see MySQL as having as many people qualified to design and monitor large systems. I am Not sure how it stacks up on the partitioning and scalibility though.