Relational algebra1
- 1. Jennifer Widom Relational Databases Relational Algebra (1) Select, project, join
- 2. Jennifer Widom Relational Algebra (1) Query (expression) on set of relations produces relation as a result
- 3. Jennifer Widom Relational Algebra (1) Examples: simple college admissions database College(cName,state,enrollment) Student(sID,sName,GPA,sizeHS) Apply(sID,cName,major,decision) cName state enr sID sName GPA HS sID cName major dec College Student Apply
- 4. Jennifer Widom Simplest query: relation name cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1) Use operators to filter, slice, combine
- 5. Jennifer Widom Select operator: picks certain rows Students with GPA>3.7 Students with GPA>3.7 and HS<1000 Applications to Stanford CS major cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 6. Jennifer Widom Project operator: picks certain columns ID and decision of all applications cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 7. Jennifer Widom To pick both rows and columns… ID and name of students with GPA>3.7 cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 8. Jennifer Widom Duplicates List of application majors and decisions cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 9. Jennifer Widom Cross-product: combine two relations (a.k.a. Cartesian product) cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 10. Jennifer Widom Cross-product: combine two relations (a.k.a. Cartesian product) Names and GPAs of students with HS>1000 who applied to CS and were rejected cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 11. Jennifer Widom Natural Join Enforce equality on all attributes with same name Eliminate one copy of duplicate attributes cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 12. Jennifer Widom Natural Join Names and GPAs of students with HS>1000 who applied to CS and were rejected cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1) Names and GPAs of students with HS>1000 who applied to CS at college with enr>20,000 and were rejected
- 13. Jennifer Widom Natural Join cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 14. Jennifer Widom Theta Join Basic operation implemented in DBMS Term “join” often means theta join cName state enr sID sName GPA HS sID cName major dec College Student Apply Relational Algebra (1)
- 15. Jennifer Widom Relational Algebra (1) Query (expression) on set of relations produces relation as a result Simplest query: relation name Use operators to filter, slice, combine Operators so far: select, project, cross-product, natural join, theta join