MSc IT UFIE8K-15-M Data Management Prakash Chatterjee Room 3P Lecture 2 : The Relational Data Model

UFIE8K15-M Data Management 2008 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.

UFIE8K15-M Data Management 2008 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.

UFIE8K15-M Data Management 2008 Components and terminology (2)

UFIE8K15-M Data Management 2008 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.

UFIE8K15-M Data Management 2008 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.first-order logicrelations closedoperators 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.

UFIE8K15-M Data Management 2008 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:UNIONINTERSECTION DIFFERENCEDIVIDE The special relational operators:RESTRICTPROJECT JOINCartesian PRODUCT

UFIE8K15-M Data Management 2008 Relational algebra operators (2)

UFIE8K15-M Data Management 2008 Relational algebra operators (3) dept – emp – salgrade example (1) dept – emp – salgrade example Department : dept (depno, dname, location) Employee : emp (empno, ename, mgr, sal, deptno) Salary Grade : salgrade (grade, losal, hisal)

UFIE8K15-M Data Management 2008 Relational algebra operators (4) dept – emp – salgrade example (2) dept table deptnodnamelocation 10AccountingNew York 20ResearchDallas 30SalesChicago 40OperationsBoston

UFIE8K15-M Data Management 2008 Relational algebra operators (5) dept – emp – salgrade example (3) empnoenamemgrsaldeptno 7369SMITH7902 £ ALLEN7698£1, WARD7698£1, JONES7839£2, MARTIN7698£1, BLAKE7839£2, CLARK7839£2, SCOTT7566£3, KING£5, TURNER7698£1, ADAMS7788£1, JAMES7698 £ FORD7566£3, MILLER7782£1, emp table

UFIE8K15-M Data Management 2008 Relational algebra operators (6) dept – emp – salgrade example (4) salgrade table gradelosalhisal 1£700.00£1, £1,201.00£1, £1,401.00£2, £2,001.00£3, £3,001.00£99,999.00

UFIE8K15-M Data Management 2008 Relational algebra operators (7) dept – emp – salgrade example (5) Restrict Subset of the Rows in a Table RESTRICT EMP WHERE sal > 2000 empnoenamemgrsaldeptno 7566JONES7839£2, BLAKE7839£2, CLARK7839£2, SCOTT7566£3, KING£5, FORD7566£3,

UFIE8K15-M Data Management 2008 Relational algebra operators (8) dept – emp – salgrade example (6) Project subset the Columns in a Table PROJECT EMP [EMPNO, SAL,DEPTNO] empnosal deptno 7369£ £1, £1, £2, £1, £2, £2, £3, £5, £1, £1, £ £3, £1,

UFIE8K15-M Data Management 2008 Relational algebra operators (9) dept – emp – salgrade example (7) Restrict-Project RESTRICT EMP WHERE SAL >2000 PROJECT EMP[EMPNO, SAL, DEPTNO] empnosaldeptno 7566£2, £2, £2, £3, £5, £3, call this EMPX Could you reverse these operations - always? ( project then restrict?)

UFIE8K15-M Data Management 2008 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 empnosalEMPX. deptno dept. Depno dnameloc 7566£2, AccountingNew York 7698£2, AccountingNew York 7782£2, AccountingNew York 7788£3, AccountingNew York 7839£5, AccountingNew York 7902£3, AccountingNew York 7566£2, ResearchDallas 7698£2, ResearchDallas 7782£2, ResearchDallas 7788£3, ResearchDallas 7839£5, ResearchDallas 7902£3, ResearchDallas

UFIE8K15-M Data Management 2008 Relational algebra operators (11) dept – emp – salgrade example (9) 7566£2, SalesChicago 7698£2, SalesChicago 7782£2, SalesChicago 7788£3, SalesChicago 7839£5, SalesChicago 7902£3, SalesChicago 7566£2, OperationsBoston 7698£2, OperationsBoston 7782£2, OperationsBoston 7788£3, OperationsBoston 7839£5, OperationsBoston 7902£3, OperationsBoston

UFIE8K15-M Data Management 2008 Relational algebra operators (12) dept – emp – salgrade example (10) Product (Cartesian product) DEPT has 4 records EMPX has 6 records so DEPT x EMPX has 24 records but not very useful

UFIE8K15-M Data Management 2008 Relational algebra operators (13) dept – emp – salgrade example (11) Equi-Join product restricted to rows which have matching common domain empno salEMPX. deptno dept. deptno dnameloc 7566£2, ResearchDallas 7698£2, SalesChicago 7782£2, AccountingNew York 7788£3, ResearchDallas 7839£5, AccountingNew York 7902£3, ResearchDallas

UFIE8K15-M Data Management 2008 Relational algebra operators (14) dept – emp – salgrade example (12) Natural Join equi-join projected with the duplicate column removed empno saldeptnodnameloc 7566£2, ResearchDallas 7698£2, SalesChicago 7782£2, AccountingNew York 7788£3, ResearchDallas 7839£5, AccountingNew York 7902£3, ResearchDallas

UFIE8K15-M Data Management 2008 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;

UFIE8K15-M Data Management 2008 Bibliography / Readings / Home based activities Bibliography - An Introduction to Database Systems (8 th ed.), C J Date, Addison Wesley 2004 An Introduction to Database Systems - Database Management Systems, P Ward & G Defoulas, Thomson 2006 Database Management Systems 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.