SQL92 tries to be a standard, and databases should adhere to that standard. Unfortunately, nobody makes forces the databases to follow the standard, so making yourself SQL92 aware is nice, (except that there are more recent standars aswell) but doesn't mean anything more than "I speak basic SQL".
Also, a DBA does a lot more than just write SQL, he also designs databases, installs servers, tunes engines.
By far the bext way to get educated in the field of complex SQL is to follow a course, and yes that will cost money. Employers like to keep their staff educated, so maybe you can get your current employer to pay for it.