The SQL Pocket Guide, 3rd Edition is a book that would be most useful to SQL users who need to work in multiple database environments and do not have an internet connection available when creating queries. It’s well-organized by topic and provides working examples where relevant, but I don’t see a place for this on my bookshelf.
My first search through the book (using the ebook version) was for the MySQL DAY() function, as I happened to be using that for a query. No relevant results were returned, although the DAYOFMONTH() function is present – so apparently synonyms are not necessarily included for all platforms. Google search provided a link to the relevant MySQL docs as the first result, and that would be my first recourse in almost every situation.
There are some value-adds in the text, for example the discussions around Oracle’s CONNECT BY feature, but unless you really do need to carry this in your pocket, you will probably be better served by a more in-depth reference for your primary database platform(s) supplemented by Google.
I’ll add that Johnathan Gennick’s previous books have taught me quite a bit. This one just seems like a solution in need of a problem.