Once you have a basic understanding of SQL, I found it really informative to go closer to the source, e.g. C. J. Date and Hugh Darwen's "A Guide to the SQL Stan

Once you have a basic understanding of SQL, I found it really informative to go closer to the source, e.g. C. J. Date and Hugh Darwen's "A Guide to the SQL Standard", or perhaps other books by them. There's a lot of incomplete and misunderstood information floating around in blog tutorials or potboiler textbooks.