An SQL injection is when a hacker attempts to abuse a bug in a site (the kind which derpibooru dev team actively looks out for and we are confident we have none of those) in order to, through bad input, manipulate the SQL query used to save or fetch the data in a way where they can damage something or alter data in a way that was not intended by the developer.
This is a server-side attack and no antivirus should be able to detect that on client-side, unless it actually detected something else and the error was very unfortunately named.
What I’m trying to say, derpibooru is safe, check your computer for viruses with a different antivirus, as your current one might have detected consequences of another virus that it failed to detect.