SQL Server is my go to relational database. It's been around for years and hugely reliable.
Microsoft's SQL Server has been around since I can remember, it's pretty stable! (Just Googled: it was initially released in 1989)
It was the first relational database that I became familiar and comfortable with.
Although I don't use SQL Server as much these days (well, apart from on all Umbraco projects) it's still a tool I'm glad to have in my toolbox.
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