Data Modelling and Databases. Exercise Session 2: Relational Model

Transcription

1 Data Modelling and Databases Exercise Session 2: Relational Model 1

2 Interactive Session Format You can watch others swim as often as you want. In the end, you can only learn it by doing it yourself. Alfons Kemper, Martin Wimmer: Übungsbuch Datenbanksysteme, 3rd Edition (2012). Do the exercises yourself! Ask questions during the exercise sessions Present your solution on the black board (we will help you!) 2

3 Administrative Aspects Exercise Sheets Not graded, not mandatory. Hand them in if you want feedback. At the end of the session By as indicated on the sheet Handout published a week before discussed. Solutions published after the Friday exercise session. Questions: sg-dmdb17@lists.inf.ethz.ch 3

4 Course Website Exercise sheets and solutions Lecture slides Announcements Contact information 4

5 Exercise 1: Keys Based on what you learned about the relational model and schemas, mark all statements below that are true: Every relation in a relational schema has at least one key. The primary key of a relation is composed of a single attribute. Secondary keys of the relation have to be composed of at least two attributes. In special cases there can be duplicate values among the keys of a relation. 5

6 Exercise 1: Keys Based on what you learned about the relational model and schemas, mark all statements below that are true: Every relation in a relational schema has at least one key. The primary key of a relation is composed of a single attribute. Secondary keys of the relation have to be composed of at least two attributes. In special cases there can be duplicate values among the keys of a relation. 6

7 Exercise 1: Scenarios Student(FirstName,LastName, Age, LegiNr) Attends(LegiNr, LectureName,...) Can you think of a real-world scenario that cannot be represented in this relational schema? 7

8 Exercise 1: Scenarios Student(FirstName,LastName, Age, LegiNr) Attends(LegiNr, LectureName,...) Can you think of a real-world scenario that cannot be represented in this relational schema? In this schema it is not possible to store two students with the same first and last name. 8

9 Exercise 2: Conversions Given the ER model, complete the table: 9

10 Exercise 2: Conversions Given the ER model, complete the table: 10

11 Exercise 2: Inserting tuples For each cardinality mark which entries can be added (in this order): (a1, b1) (a1, b2) (a2, b1) (a2, b2) (a1, b1) 11

12 Exercise 2: Inserting tuples M:N 1:N N:1 1:1 (a1, b1) (a1, b1) (a1, b1) (a1, b1) (a1, b2) (a1, b2) (a1, b2) (a1, b2) (a2, b1) (a2, b1) (a2, b1) (a2, b1) (a2, b2) (a2, b2) (a2, b2) (a2, b2) (a1, b1) (a1, b1) (a1, b1) (a1, b1) 12

13 Exercise 3: Finish conversion Which one of the following problem descriptions could have lead to the ER model above? We are modeling a voleyball tournament. A volleyball game is played by two teams. Multiple referees make sure during a single game that the rules are respected. A volleyball game in a tournament is played by two teams. A referee has to be present at each game to make sure the rules are respected. Two teams play volleyball in a tournament. In each game, one has the role of home team and the other one has the role of away team. Each play is supervised by at most one referee. 13

14 Exercise 3: Finish conversion Which one of the following problem descriptions could have lead to the ER model above? We are modeling a voleyball tournament. A volleyball game is played by two teams. Multiple referees make sure during a single game that the rules are respected. A volleyball game in a tournament is played by two teams. A referee has to be present at each game to make sure the rules are respected. Two teams play volleyball in a tournament. In each game, one has the role of home team and the other one has the role of away team. Each play is supervised by at most one referee. 14

15 Exercise 3: Fix the schema 15

16 Exercise 3: Fix the schema Correct key: Home, Away 16

17 Exercise 4: Weak Entities 17

18 Exercise 4: Weak Entities Q: Would it be possible to use less attributes as keys in any of the relations? 18

19 Exercise 5: Employees [ $30000 ) $45000 time All other dates work the same way. 19

20 Exercise 5: Employees Two departments with the name "Engineering" could exist at the same time. No employee can work at a department and manage another one at the same time. An employee could be employed at two departments at the same time. For the entries in the dept_manager relation the to_date cannot be before the from_date. Given an employee we can identify exactly one department where (s)he works. An employee could collect more than one salary at the same time. Some entries in salaries could have no from_date and no employee associated to them. 20

21 Exercise 5: Employees Two departments with the name "Engineering" could exist at the same time. No employee can work at a department and manage another one at the same time. An employee could be employed at two departments at the same time. For the entries in the dept_manager relation the to_date cannot be before the from_date. Given an employee we can identify exactly one department where (s)he works. An employee could collect more than one salary at the same time. Some entries in salaries could have no from_date and no employee associated to them. 21

22 Exercise 6: Schema Training See exercise sheet 22