Rrelational algebra in dbms overview
- 1. Relational Algebra 1 By:-Gourav Kottawar
- 2. Relational Algebra is : the formal description of how a relational database operates an interface to the data stored in the database itself the mathematics which underpin SQL operations 2 By:-Gourav Kottawar
- 3. Operators There are two groups of operations: Mathematical set theory based relations: UNION, INTERSECTION, DIFFERENCE, and CARTESIAN PRODUCT. Special database operations: SELECT (not the same as SQL SELECT), PROJECT, and JOIN. 3 By:-Gourav Kottawar
- 4. Relational PROJECT The PROJECT operation is used to select a subset of the attributes of a relation by specifying the names of the required attributes. For example, to get a list of all employees surnames and employee numbers: PROJECTsurname,empno(employee) Project is denoted by (π ) 4 By:-Gourav Kottawar
- 5. Relational SELECT SELECT is used to obtain a subset of the tuples of a relation that satisfy a select condition. For example, find all employees born after 1st Jan 1950: SELECT dob '01/JAN/1950'(employee) Select is denoted by (sigma) 5 By:-Gourav Kottawar
- 6. SELECT and PROJECT SELECT and PROJECT can be combined together. For example, to get a list of employee numbers for employees in department number 1: Figure : Mapping select and project 6 By:-Gourav Kottawar
- 7. UNION Example 7 By:-Gourav Kottawar
- 8. INTERSECTION Example 8 By:-Gourav Kottawar
- 9. DIFFERENCE Example 9 By:-Gourav Kottawar
- 10. CARTESIAN PRODUCT The Cartesian Product is also an operator which works on two sets. It is sometimes called the CROSS PRODUCT or CROSS JOIN. It combines the tuples of one relation with all the tuples of the other relation. Cartesian Product (X). 10 By:-Gourav Kottawar
- 12. JOIN Operator JOIN is used to combine related tuples from two relations: In its simplest form the JOIN operator is just the cross product of the two relations. As the join becomes more complex, tuples are removed within the cross product to make the result of the join more meaningful. JOIN allows you to evaluate a join condition between the attributes of the relations on which the join is undertaken. The notation used is R JOINjoincondition S 12 By:-Gourav Kottawar