Any one in detail, but be aware of the sort of differences found in the others. Which one should be your core one depends on what projects you wish to work on.

Any one in detail, but be aware of the sort of differences found in the others. Which one should be your core one depends on what projects you wish to work on. My DayJob is an MS shop to SQL Server's TSQL is my area of expertise, but I know more-or-less what isn't supported or is handled differently in other common places (core postgres, mysql/mariadb, sqlite). In some places you may end up being more completely fluent in multiple rather than just one. The key is understanding the concepts (set based operations rather than thinking procedurally, recursive queries, window functions, how query planners commonly work so you can optimise for them) rather than specific syntax which is always easy to lookup.