UFCEKG-20-2 Data, Schemas & Applications. Lecture 8 Database Theory & Practice (2) : The Relational Data Model. Origins of the Relational Model. The relational model was developed by EF Codd in the early 1970s.
Lecture 8Database Theory & Practice (2) : The Relational Data Model
Informally, a relational system is a system in which:
1. The data is perceived by the user as tables (and nothing but tables).
2. The operators available to the user for (e.g.) retrieval are operators that derive “new” tables from "old" ones. For example, there is one operator, restrict, which extract a subset of the rows of a given table, and another, project, which extracts a subset of columns - and a row subset and a column subset of a table can both be regarded in turn as tables in their own right.
The model uses terminology taken from mathematics, particularly set theory and predicate logic.
Basic terminology used in relational theory includes:
Department : dept (depno, dname, location)
Employee : emp (empno, ename, mgr, sal, deptno)
Salary Grade : salgrade (grade, losal, hisal)
RESTRICT EMP WHERE sal > 2000
PROJECT EMP [EMPNO, SAL,DEPTNO]
RESTRICT EMP WHERE SAL >2000
PROJECT EMP[EMPNO, SAL, DEPTNO]
call this EMPX
Could you reverse these operations - always? ( project then restrict?)
PRODUCT DEPT with EMPX
continued from last page :
DEPT has 4 records
EMPX has 6 records
so DEPT x EMPX has 24 records
but not very useful
product restricted to rows which have matching common domain
equi-join projected with the duplicate column removed