1. Lecture 16 : The Relational Data Model
MSc IT UFCEKG-20-2 DSA Prakash Chatterjee Room 3P16
Lecture 16 : The Relational Data Model

2. Origins of the Relational Model
The relational model was developed by EF Codd in the early 1970s.
Commercial systems based on the relational model appeared in the late 1970s.
At present there are several hundred relational DBMSs and most computer vendors support 'relational' software.
Examples of well-known products include Oracle, DB2, Sybase, MySQL, MS.SQL Server and MS Access.
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.

3. Components and terminology (1)
The model uses terminology taken from mathematics, particularly set theory and predicate logic. Basic terminology used in relational theory includes:
relation - this corresponds to a table or flat file with columns and rows
tuple - a row of a relation
attribute - a named column of a relation
domain - the set of allowable values for one or more attributes
degree of a relation - the number of attributes it contains
cardinality of relation - the number of tuples it contains.

4. Components and terminology (2)

5. Properties of relations
There is only one data structure in the relational data model - the relation.
Every relation and every attribute within a relation must have a distinct name.
Attribute (column) values of a relation are atomic (i.e. single valued).
All values in an attribute (column) are taken from same domain.
The ordering of columns in a relation is not significant.
Duplicate tuples (rows) are not allowed (e.g. each row in a relation must be distinct).
The ordering of tuples (rows) and attributes (columns) is not significant.

6. Relational algebra & relational calculus
Relational algebra (ra) and relational calculus (rc) are both formal (mathematically based) languages defined by EF Codd.
ra & rc are logically equivalent languages. ra is “procedural” and rc is “declarative” in nature.
ra and rc are the formal grounding of the relational database model and illustrate the basic operations required by any data manipulation language such as SQL.
Relational algebra is an offshoot of first-order logic, is a set of relations closed under operators. Operators operate on one or more relations to yield a relation.
The “closure” property relates to the fact that from any given relational operation another relation is output - it os often refereed to as the “relations in – relations out” property.

7. Relational algebra operators (1)
Each relational operator takes one or more relations as its input and produces a new relation as output (closure). Codd originally defined eight operators, in two classes:
Set operators:
UNION
INTERSECTION
DIFFERENCE
DIVIDE
The special relational operators:
RESTRICT
PROJECT
JOIN
Cartesian PRODUCT

8. Relational algebra operators (2)

9. Relational algebra operators (3) dept – emp – salgrade example (1)
Department : dept (depno, dname, location)
Employee : emp (empno, ename, mgr, sal, deptno)
Salary Grade : salgrade (grade, losal, hisal)

10. Relational algebra operators (4) dept – emp – salgrade example (2)
dept table
deptno
dname
location 10
Accounting
New York 20
Research
Dallas 30
Sales
Chicago 40
Operations
Boston

11. Relational algebra operators (5) dept – emp – salgrade example (3)
emp table
empno
ename
mgr
sal
deptno
7369
SMITH
7902
£800.00 20
7499
ALLEN
7698
£1,600.00 30
7521
WARD
£1,250.00
7566
JONES
7839
£2,975.00
7654
MARTIN
BLAKE
£2,850.00
7782
CLARK
£2,450.00 10
7788
SCOTT
£3,000.00
KING
£5,000.00
7844
TURNER
£1,500.00
7876
ADAMS
£1,100.00
7900
JAMES
£950.00
FORD
7934
MILLER
£1,300.00

12. Relational algebra operators (6) dept – emp – salgrade example (4)
salgrade table
grade
losal
hisal 1
£700.00
£1,200.00 2
£1,201.00
£1,400.00 3
£1,401.00
£2,000.00 4
£2,001.00
£3,000.00 5
£3,001.00
£99,999.00

13. Relational algebra operators (7) dept – emp – salgrade example (5)
Restrict Subset of the Rows in a Table
RESTRICT EMP WHERE sal > 2000
empno
ename
mgr
sal
deptno
7566
JONES
7839
£2,975.00 20
7698
BLAKE
£2,850.00 30
7782
CLARK
£2,450.00 10
7788
SCOTT
£3,000.00
KING
£5,000.00
7902
FORD

14. Relational algebra operators (8) dept – emp – salgrade example (6)
Project
subset the Columns in a Table
PROJECT EMP [EMPNO, SAL,DEPTNO]
empno
sal
deptno
7369
£800.00 20
7499
£1,600.00 30
7521
£1,250.00
7566
£2,975.00
7654
7698
£2,850.00
7782
£2,450.00 10
7788
£3,000.00
7839
£5,000.00
7844
£1,500.00
7876
£1,100.00
7900
£950.00
7902
7934
£1,300.00

15. Relational algebra operators (9) dept – emp – salgrade example (7)
Restrict-Project
RESTRICT EMP WHERE SAL >2000
PROJECT EMP[EMPNO, SAL, DEPTNO]
empno
sal
deptno
7566
£2,975.00 20
7698
£2,850.00 30
7782
£2,450.00 10
7788
£3,000.00
7839
£5,000.00
7902
call this EMPX
Could you reverse these operations - always? ( project then restrict?)

16. Relational algebra operators (10) dept – emp – salgrade example (8)
Product
combine each row of one table with each row of the other
PRODUCT DEPT with EMPX
empno
sal
EMPX.
deptno
dept.
Depno
dname
loc
7566
£2,975.00 20 10
Accounting
New York
7698
£2,850.00 30
7782
£2,450.00
7788
£3,000.00
7839
£5,000.00
7902
Research
Dallas

17. Relational algebra operators (11) dept – emp – salgrade example (9)
7566
£2,975.00 20 30
Sales
Chicago
7698
£2,850.00
7782
£2,450.00 10
7788
£3,000.00
7839
£5,000.00
7902 40
Operations
Boston

18. Relational algebra operators (12) dept – emp – salgrade example (10)
DEPT has 4 records
EMPX has 6 records
so DEPT x EMPX has 24 records
but not very useful

19. Relational algebra operators (13) dept – emp – salgrade example (11)
Equi-Join
product restricted to rows which have matching common domain
empno
sal
EMPX.
deptno
dept.
dname
loc
7566
£2,975.00 20
Research
Dallas
7698
£2,850.00 30
Sales
Chicago
7782
£2,450.00 10
Accounting
New York
7788
£3,000.00
7839
£5,000.00
7902

20. Relational algebra operators (14) dept – emp – salgrade example (12)
Natural Join
equi-join projected with the duplicate column removed
empno
sal
deptno
dname
loc
7566
£2,975.00 20
Research
Dallas
7698
£2,850.00 30
Sales
Chicago
7782
£2,450.00 10
Accounting
New York
7788
£3,000.00
7839
£5,000.00
7902

21. Basic SQL SELECT * FROM EMP WHERE SAL > 2000;
SELECT ENAME,SAL,DEPTNO FROM EMP;
SELECT ENAME,SAL,DEPTNO FROM EMP WHERE SAL > 2000;
SELECT * FROM EMP, DEPT WHERE SAL > 2000;
SELECT * FROM EMP,DEPT WHERE SAL > 2000 AND EMP.DEPTNO = DEPT.DEPTNO;
SELECT EMPNO, SAL, DEPTNO, DNAME FROM EMP,DEPT WHERE SAL > 2000 AND EMP.DEPTNO = DEPT.DEPTNO;

22. Bibliography / Readings / Home based activities
An Introduction to Database Systems (8th ed.), C J Date, Addison Wesley 2004
Database Management Systems, P Ward & G Defoulas, Thomson 2006
Readings
Introduction to SQL’ McGraw-Hill/Osbourne (handout)
Home based activities
Ensure you download xampp and install on home PC or laptop (if you have a slow home internet connection – download to data key or CD here at UWE)
Copy the SQL Workbook onto your data key or CD.
Import the tables from the SQL Workbook into your home MySQL DB. Begin working through some of the query examples in the workbook using PHPMyAdmin.