A DataBase built around SetTheory and SQL.
A relation (AKA a table) is a set of rows. A row is a sequence of fixed length fields (AKA columns or attributes). A column is uniquely identified within the relation by a name. A row is uniquely identified within the relation by the values of it's primary key. The default primary key, which is a non-strict subset of the columns of the table.
A secondary key is a non-strict subset of the columns of any table that correspond to the primary key of another table.
A relation differs from a SpreadSheet in that the rows and columns are unordered.
SQL set-oriented is a language designed to build, maintain and query such DataBases.
RelationalDataBase quality revolves around five normal forms:
These aren't very good explainations, but I didn't want to cut and paste. There are very large number of books and websites on relational database design.
There are very few real-world RelationalDataBases, most of the databases which we thing of as RelationalDataBases (Oracle, MySQL, PostgreSQL and DB2) are actually PostRelationalDataBases.
10 pages link to RelationalDataBase: