I am a computer engineer with 21 years experience and 14 years experience with DBs. I've done many E/R diagrams - if you already have the tables it's a very easy thing. I could show you some of the work I've done here. Then I've done bigger ones for larger companies. I also like to draw it up so it has some integrity, with the right engine type, foreign keys where appropriate, enforcement of non null, indices, and so on.