Path: news.cambrium.nl!textnews.cambrium.nl!feeder2.cambriumusenet.nl!feed.tweaknews.nl!209.197.12.246.MISMATCH!nx02.iad01.newshosting.com!newshosting.com!news.glorb.com!Xl.tags.giganews.com!border1.nntp.dca1.giganews.com!nntp.giganews.com!local2.nntp.dca.giganews.com!nntp.megapath.net!news.megapath.net.POSTED!not-for-mail NNTP-Posting-Date: Thu, 11 Dec 2014 20:13:17 -0600 Date: Thu, 11 Dec 2014 21:13:16 -0500 From: "James K. Lowden" Newsgroups: comp.databases.theory Subject: Re: relational algrabra division? Message-Id: <20141211211316.377bdab6.jklowden@speakeasy.net> References: X-Newsreader: Sylpheed 3.0.2 (GTK+ 2.20.1; x86_64--netbsd) Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Lines: 24 X-Usenet-Provider: http://www.giganews.com NNTP-Posting-Host: 209.166.36.60 X-Trace: sv3-BWSoAZ9sqLRitjpIJlYvuoQ7M5IURd/EXPbyLZqCBzByMkKFTwlL3qBmnRyyVqZdEC0EXTfdqQpercL!To6UK/X6IVw6+Gjz1aHqvIB+Fi8G8611KsJSbrbh9my4Wk3H41+APER2wyD0JjYvuoD0h7LNtAUO!SDyU64TKRDJRxZTvoA6XfP5reMYCOCjt X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 X-Original-Bytes: 1610 Xref: news.cambrium.nl On Thu, 11 Dec 2014 13:04:27 -0500 ruben safir wrote: > What class of query is this called and how is it generlized > mathmatically. A join is called a "cross product" and may be thought of as multiplication. Relational division is its inverse: C =3D A =D7 B B =3D C =F7 A It is not well represented in SQL, unfortunately. =20 >> sometimes occurs in database applications.=20 >=20 > Really? What kind? > (e) Retrieve the names of employees who work on every project That kind! =20 --jkl