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Understanding Relational Algebra in Database Management Systems

This lecture covers the fundamentals of relational algebra, a formal language for the relational model in database management systems. It explains key operations such as selection, projection, join, and set operations like union, intersection, and set difference. The lecture provides examples of relational algebra expressions and demonstrates how to formulate selections based on specific conditions, such as filtering students by GPA or querying employee details based on salary and department. Understanding these concepts is crucial for effective database querying and manipulation.

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Understanding Relational Algebra in Database Management Systems

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  1. Entity-Relationship ModelCont….. CST203-2 Database Management Systems Lecture 5

  2. There are 2 formal languages for relational model • Relational algebra • Relational calculus

  3. Relational Algebra

  4. What is relational algebra? • The result? • Sequence of relational algebra • Divided into 2 groups

  5. Special operations • Selection • Projection • Join • Rename • Set operations • Union • Intersection • Set difference

  6. Similar to normal algebra

  7. SELECT operation σσ<Selection Condition>(R) • σ – SELECT operator • Selection Condition : Boolean expression • R : relation • If more selection conditions, • Use ‘OR’, ‘AND’ and ‘NOT’

  8. Horizontal partition • Select the students who has the GPA greater than 3.5 σσGPA > 3.5 (STUDENT) Student Table

  9. +++++++++

  10. σ<Condition1>(σ<Condition2>(R)) = σ<Condition2>(σ<Condition1>(R)) σ<Cond1>(σ<Cond2>(σ<Cond3>(R))) = σ<Cond1> AND <Cond2> AND <Cond3>(R)

  11. Assignment • Write the relational algebra for selecting all details whose department is 4 and whose salary is greater than 30,000 • Write it in another way

  12. PROJECT operation • If want to choose a subset of the columns in a relation and discards the rest, • Use Π ΠName, GPA (Student) Π<attribute list>(R)) Π<list1>(Π<list2>(R))) = Π<list>(R))

  13. Assignment • Write the relational algebra for selecting Name, Eno, and Address

  14. Sequence of operations STU_1stCLASSΠname(σGPA > 3.5 (STUDENT))

  15. Assignment • Write the relational algebra for selecting Name, Eno, and Address of all male employees whose salary is greater than 20,000

  16. CARTESIAN PRODUCT

  17. INNER JOIN

  18. UNION operation Result 1 Πname(σGPA > 3.5 (STUDENT)) Result 2 Πname(σ(GPA > 2.5 AND GPA < 3.5) (STUDENT)) Result Result1 υ Result 2

  19. Result 1 • Result 2 • Result

  20. INTERSECTION operation Result 1 Πname(σGPA > 3.5 (STUDENT)) Result 2 Πname(σ(GPA > 2.5) (STUDENT)) Result Result1 Result 2 υ

  21. Result 1 • Result 2 • Result

  22. SET DIFFERENCE • Also called as MINUS Result Result1 – Result2

  23. Result 1 • Result 2 • Result

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