1 / 23

Review

Review. Exam Su 3:30PM - 6:30PM 2010/12/12 Room C9000. Algebra. Relational Algebra Operations. Basic operations Selection ( ) Selects a subset of rows from relation. Projection ( ) Deletes unwanted columns from relation.

carson-rosales
Download Presentation

Review

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Review

  2. Exam • Su 3:30PM - 6:30PM 2010/12/12 • Room C9000

  3. Algebra

  4. Relational Algebra Operations • Basic operations • Selection ( ) Selects a subset of rows from relation. • Projection( ) Deletes unwanted columns from relation. • Cartesian product( ) Combinetwo relations. • Set-difference ( ) Tuples in relation 1, but not in relation 2. • Union( ) Tuples in relation 1 or in relation 2.

  5. Functional Dependency

  6. Functional Dependencies (FDs) • A functional dependency XY holds over relation R if, for every allowable instance r of R: • given two tuples in r, if the X values agree, then the Y values must also agree. (X and Y are sets of attributes.) • An FD is a statement about all allowable relations. • Must be identified based on semantics of application. • Given some allowable instance r1 of R, we can check if it violates some FD f, but we cannot tell if f holds over R! • K is a candidate key for R means that KR • However, KR does not require K to be minimal!

  7. Wages Example (Contd.) • Problems due to R W : • Update anomaly: Can we change W in just the 1st tuple of SNLRWH? • Insertion anomaly: What if we want to insert an employee and don’t know the hourly wage for his rating? • Deletion anomaly: If we delete all employees with rating 5, we lose the information about the wage for rating 5! Hourly_Emps2

  8. Reasoning About FDs (Contd.) • Couple of additional rules (that follow from AA): • Union: If X  Y and X  Z, then X  YZ • Decomposition: If X  YZ, then X  Y and X  Z • Example: Contracts(cid,sid,jid,did,pid,qty,value), and: • C is the key: C  CSJDPQV • Project purchases a part using single contract: JPC • Dept purchases at most a part from a supplier:SD P • JP  C, C  CSJDPQV imply JP  CSJDPQV • SD  P implies SDJ  JP • SDJ  JP, JP  CSJDPQV imply SDJ  CSJDPQV

  9. Suppose that relation R contains attributes A1 ... An. A decompositionof R consists of replacing R by two or more relations such that: • Each new relation scheme contains a subset of the attributes of R (and no attributes that do not appear in R), and • Every attribute of R appears as an attribute of one of the new relations. • e.g., A  B, ABCD  E, EF  GH, ACDF  EG has the following minimal cover: • A  B, ACD  E, EF  G and EF  H

  10. Examples Does A B hold? Don’t know Does BC  A hold? Does not hold (first two tuples) Does B  C hold? Does not hold (last 2 tuples)

  11. Examples • Relation: ABCDE • Dependencies: • Decompose to?

  12. Examples • Relation: ABCD • Dependencies: • What is the candidate key?

  13. Examples • Relation: ABCD • Dependencies: • What is the candidate key?

  14. Examples • Relation: ABCD • Dependencies: • What is the candidate key?

  15. Examples • Relation: ABCD • Dependencies: • What is the candidate key?

  16. Examples • Suppose you are given a relation R(A,B,C,D) • Dependencies • What is the candidate key? • Decomposed into ACD and BC. Good?

  17. Examples • Suppose you are given a relation R(A,B,C,D) • Dependencies • What is the candidate key? • Decomposed into ABC and AD. Good?

More Related