Stack Overflow provided a great tool for creating charts. This is mine for “PostgreSQL vs MySQL vs Oracle vs MSSQL”.
PS Oracle and MSSQL have some additional tags depending on version. However, I’m sure we may not include them into statistics because the main tag is always used IMO.
Pasha Golub's Blog 
Not sure what conclusions can be drawn, if any. Perhaps not the point, though. What I mean is that how did StackOverflows total body of questions expand over this time frame? For example the decline of SQL Server could be due to the overall amount of different subjects increasing, making SQL Server questions a smaller part of the total. Or perhaps many common question had already been asked and there was therefore less need to ask new questions. On the other hand, the rise in MySql from 2008 to 2010 perhaps means that this was the time the mysql community of users discovered StackOverflow, making it more useful for them and therefore attracting further mysql questions (network effect). I’m wondering if a graph showing absolute numbers would be more interesting, or a graph showing percentage of the total number of database-related or sql-related questions (instead of all questions.).
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