CityDev wrote: > Just to kill time over coffee - what do you take the word to mean? > > I've just been reading a 1991 James Martin book on Object Orientation and he > was using it to talk about links between entities. Chris Date was very > specific that a relation was essentially a table. Mainly however, people > seem to use the word to describe the connections you can make by performing > joins between tables. What do you think is 'correct'? How did the other > meaning gain currency?
In the context of relational databases or mathematics, you want to use Chris Date's meaning, which comes from mathematics dating from before we had computers as we know them today. A relation is a set of tuples where all tuples are of the same degree and have the same set of attribute names/types. For example, you can have a "people" relation where each tuple represents a "person" and every tuple has 3 attributes, ["name", "birthdate", "address"]. A relation gets its name from that, for every tuple in it, each attribute value is related to the other attribute values. For example, in a "person" tuple ["Joe", "Feb 17, 1989", "53 Cherry Dr."], the "Joe" is related to "53 Cherry Dr."] and so on. A relational database is called that because it consists of a set of relations, each of which having a name. SQL uses the terms ["rowset","row","column/field"] to refer to a ["relation","tuple","attribute"], and the term "table" to refer to a relation-typed variable, such being what a persisting database consists of. A relational join is an operation that takes several relation values (rowsets) as input and combines them to yield another relation value (rowset) as output, such that member tuples (rows) are matched up with each other and catenated into a new set of tuples (rows). So the term relation refers both to the contents of a SQL table *and* to a process of connecting tables. Object orientation has nothing to do with all this per se, though objects can easily be mapped to tuples. -- Darren Duncan _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
