CS
Chapter08ARelationalAlgebra (1).pptx

This operation will take the union of tuples in two

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This operation will take the union of tuples in two relations R(X, Y) and S(X, Z) that are partially compatible , meaning that only some of their attributes, say X, are type compatible. The attributes that are type compatible are represented only once in the result, and those attributes that are not type compatible from either relation are also kept in the result relation T(X, Y, Z). Relational Algebra and Calculus 62
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Outer Union Operations Example: An outer union can be applied to two relations whose schemas are STUDENT(Name, SSN, Department, Advisor) and INSTRUCTOR(Name, SSN, Department, Rank). Tuples from the two relations are matched based on having the same combination of values of the shared attributes— Name, SSN, Department. If a student is also an instructor, both Advisor and Rank will have a value; otherwise, one of these two attributes will be null. The result relation STUDENT_OR_INSTRUCTOR will have the following attributes: STUDENT_OR_INSTRUCTOR (Name, SSN, Department, Advisor, Rank) Relational Algebra and Calculus 63
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Examples of Queries in Relational Algebra: Procedural Form Q1: Retrieve the name and address of all employees who work for the ‘Research’ department. RESEARCH_DEPT DNAME=’Research’ (DEPARTMENT) RESEARCH_EMPS (RESEARCH_DEPT DNUMBER= DNOEMPLOYEE EMPLOYEE) RESULT FNAME, LNAME, ADDRESS (RESEARCH_EMPS) Q6: Retrieve the names of employees who have no dependents. ALL_EMPS SSN (EMPLOYEE) EMPS_WITH_DEPS ( SSN ) ESSN ( DEPENDENT ) EMPS_WITHOUT_DEPS (ALL_EMPS - EMPS_WITH_DEPS) RESULT LNAME, FNAME (EMPS_WITHOUT_DEPS * EMPLOYEE) Relational Algebra and Calculus 64
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Examples of Queries in Relational Algebra: Single expressions As a single expression, these queries become: Q1: Retrieve the name and address of all employees who work for the ‘Research’ department. Fname, Lname, Address (σ Dname= ‘Research’ (DEPARTMENT Dnumber=Dno(EMPLOYEE)) Q6: Retrieve the names of employees who have no dependents. Lname, Fname (( Ssn (EMPLOYEE) − ρ Ssn ( Essn (DEPENDENT))) EMPLOYEE) Relational Algebra and Calculus 65
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