What query language is most common for e-journal searches?
Academic databases are built for people who have no idea what a 'query language' is. I agree that the world would be a much better place if everyone knew a little bit of SQL, but it's not like that.

The only exception that I know is Zentralblatt, an academic database specialized to mathematics which includes a sort of text-based query language.

Every other academic database that I have worked with uses custom-made input masks where you can select manually fields to search on and logical operators; they look like this example, again from Zentralblatt.