First of all, I would like to thank my colleagues assisting me in creating and maintaining the SQL-EX.RU web site, namely A.V. Maystrenko, O.V. Lyssenko, V.U. Kalinkin,
D.I.Valuyev, V.Dolgopolov, P.A. Kurochkin, E.A.Krasovskiy, V.V.Koptel'tsev; without their support, neither the site, nor
this book would have been possible. The author also wishes to express
his appreciation to V.I. Gershovich, who was the first to come up with
the idea of writing this book, and to his coauthors having written particular topics of this book - А.Yu. Bezhayev, D.N. Chebykin, E.A.Krasovskiy. I’m also grateful to all exercise
authors, contributors to the web resource, as well as many site visitors
whose mistakes are analyzed here. I’m not going to list all their names
here since they can be found on www.sql-ex.ru.
Two database schemas along with a number of corresponding exercises have been adopted from the book by J.Ulman and J.Widom , to whom the author also wants to express his gratitude.