Assignment 7: Integrity Constraints

Transcription

1 Data Modelling and Databases Exercise dates: April 19, 2018 Ce Zhang, Gustavo Alonso Last update: April 19, 2018 Spring Semester 2018 Head TA: Ingo Müller Assignment 7: Integrity Constraints This assignment will be discussed during the exercise slots indicated above. If you want feedback for your copy, hand it in during the lecture on the Wednesday before (preferably stapled and with your address). You can also annotate your copy with questions you think should be discussed during the exercise session. If you have questions that are not answered by the solution we provide, send them to Renato 1 Foreign Keys and Constraints: Theory 1. Given a database with multiple tables, which of the following constraints can be used in a way to ensure, or will by definition not allow NULL values to be inserted? UNIQUE NOT NULL FOREIGN KEY PRIMARY KEY CHECK 2. Are the following statements true or false? Place a checkmark in the correct ones. In principle a CHECK constraint could be added to a table that makes it impossible to insert any data. A PRIMARY KEY attribute can have up to one NULL value. The PRIMARY KEY constraint can be augmented with an ON DELETE SET NULL. The ON DELETE CASCADE constraint specified for an attribute in a table leads to record deletions in the referenced tables. The CHECK constraint cannot alter values in a table.

2 3. In the lecture, we have seen the following ways to ensure consistency in a database: NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, and DEFAULT. Fill in the following grid answering which of these applies to which use case. Mark cases that apply with an. Explain why. Consistency Keyword NOT NULL UNIQUE PRIMARY KEY FOREIGN KEY CHECK DEFAULT Ensure referential consistency between relations. Can be used to... Enforce that each row has some attribute. Limit the domain of values that can be stored in an attribute. Enforce a constraint based on the content of a different table. Enforce a constraint based on two different attributes. Uniquely identify records.

3 2 Foreign Keys and Constraints: SQL 1 1. Consider the following schema creation statement for a university: CREATE TABLE student ( id INT PRIMARY KEY, age INT NOT NULL CREATE TABLE lecture ( id INT PRIMARY KEY, name VARCHAR (32) NOT NULL CREATE TABLE attends ( sid INT, lid INT, CONSTRAINT fk1 FOREIGN KEY ( sid ) REFERENCES student ( id) ON DELETE CASCADE ON UPDATE CASCADE, CONSTRAINT fk2 FOREIGN KEY ( lid ) REFERENCES lecture ( id) ON DELETE NO ACTION ON UPDATE SET NULL Initially, the three tables contain the following entries: student id age id lecture name 2 Databases 3 Literature 4 Biology attends sid lid Fill in the table, showing how many tuples the tables contain after executing each set of SQL statements. Assume that each set of SQL statements is executed on the initial state of the database above and that individual SQL statements are executed in a separate transaction. SQL student lecture attends INSERT INTO lecture VALUES (1, ' Math I' INSERT INTO lecture VALUES (2, ' Math II ' INSERT INTO student VALUES (9, 30) ; INSERT INTO attends VALUES (9, 2 DELETE FROM lecture WHERE id >2; UPDATE lecture SET id =99 WHERE id <=2; DELETE FROM student WHERE age > ( SELECT AVG ( age ) FROM student

4 2. Consider the following schemas: Schema A Schema B Schema C tab1_key INT NOT NULL UNIQUE tab1_key INT NOT NULL UNIQUE tab1_key INT NOT NULL tab2_ref INT NOT NULL, FOREIGN KEY ( tab2_ref ) REFERENCES tab1 ( tab1_key ) ON DELETE RESTRICT ON UPDATE NO ACTION tab2_ref INT NOT NULL, FOREIGN KEY ( tab2_ref ) REFERENCES tab1 ( tab1_key ) tab2_ref INT NOT NULL Schema D tab1_key INT PRIMARY KEY tab2_key INT PRIMARY KEY, tab2_ref INT NOT NULL, CONSTRAINT tab2_fk FOREIGN KEY ( tab2_ref ) REFERENCES tab1 ( tab1_key ) ON DELETE CASCADE ON UPDATE SET NULL CREATE TABLE tab3 ( tab3_ref INT, CONSTRAINT tab3_fk FOREIGN KEY ( tab3_ref ) REFERENCES tab2 ( tab2_key ) ON DELETE CASCADE ON UPDATE CASCADE Schema E tab1_key INT PRIMARY KEY tab2_key INT PRIMARY KEY, tab2_ref INT NOT NULL, FOREIGN KEY ( tab2_ref ) REFERENCES tab1 ( tab1_key ) ON DELETE CASCADE ON UPDATE CASCADE CREATE TABLE tab3 ( tab3_ref INT, FOREIGN KEY ( tab3_ref ) REFERENCES tab2 ( tab2_key ) ON DELETE SET NULL ON UPDATE CASCADE For schemas A, B and C, the tables tab1 and tab2 initially have the following content: tab1_key 1 2 tab2_key 1 2 For schemas D and E, the tables tab1, tab2 and tab3 initially have the following content: tab1_key 1 2 tab2_key tab2_ref tab3_ref 10 20

5 How many records will each schema hold (summed over all of its tables) after executing individually one of the following statements? Fill in the table below with the resulting number of records or mark an execution error with in the corresponding cell. Statements: I. UPDATE tab2 SET tab2_ ref =3 WHERE tab2_ ref =1; II. UPDATE tab1 SET tab1_ key =3 WHERE tab1_ key =1; III. DELETE FROM tab1 WHERE tab1_ key =1; A B C D E I II III 3 Foreign Keys and Constraints: SQL 2 For this exercise, you need exercise07-2_schemas.sql and exercise07-2_queries.sql. First, take a look at exercise07-2_schemas.sql. It contains four different schema variations of the Employees database. Furthermore, exercise07-2_queries.sql contains 16 queries that can be executed on top of these schemas and one modification of each of these schemas that should be executed before running queries The queries as well as schemas are numbered in the.sql files the same way as they are numbered in this document. 1. Run all combinations of schemas and queries and mark your findings in the table below. Use an to note whether a query failed to run. What is the reason that this query/schema combination failed? Schema a) b) c) d) Queries For queries 15 and 16, list the row count of the following query (if running): SELECT * FROM dept_manager Schema a) b) c) d) Queries 15 16

6 4 Foreign Keys and Constraints: SQL 3 If you want to test your solutions for this exercise, set up the tables in exercise07-3.sql in a new database. They follow this relational schema: readers: (reader_id, last_name, first_name, birth_date) books: (isbn, title, author_id, publisher_id, number_of_pages, year_of_publication) publishers: (publisher_id, name, address) categories: (category_name, subcategory_of) authors: (author_id, first_name, last_name) copies: (isbn, copy_id, shelf_no, position) borrows: (reader_id, isbn, copy_id, return_date) 1. Mark all of the following statements which are true regarding the use of integrity constraints. If primary keys are not used, two categories with the same name can exist. If there is a foreign key reference FOREIGN KEY (isbn) REFERENCES books(isbn) ON DELETE NO ACTION from copies to books, removing a book from the database without removing its copies is possible. A foreign key reference FOREIGN KEY (subcategory_of) REFERENCES categories(category_name) ON UPDATE CASCADE from categories to categories enforces that changing a category s name will cause all subcategory_of values with the original name to change to the new name. If there is a foreign key reference FOREIGN KEY(reader_id)REFERENCES readers(reader_id)on DELETE NO ACTION from borrows to readers, a malicious reader with access to the database could write a query to delete all of her borrowing records. 2. What foreign key constraints are necessary and which strategies (ON UPDATE/ON DELETE) have to be applied when changing them? Please give an explanation why you chose a particular strategy. 3. According to your solution to the previous task: What is the implication of deleting a publisher, i.e., removing a row from the publishers relation? What is the consequence of updating a reader_id? 4. Enforce that all the readers must be older than 18 years.

7 5 Employee Database Constraints Consider the employee database from previous exercises: departments: (dept_no:string, dept_name:string) dept_emp: (emp_no:int, dept_no:string, from_date:date, to_date:date) dept_manager: (emp_no:int, dept_no:string, from_date:date, to_date:date) employees: (emp_no:int, birth_date:date, first_name:string, last_name:string, gender:enum, hire_date:date) salaries: (emp_no:int, salary:int, from_date:date, to_date:date) title: (emp_no:int, title:string, from_date:date, to_date:date) How would you add the following constraints, when you create the corresponding table in SQL? Write a complete CREATE TABLE query with the required constraints. 1. The start date of salary has to be before the end date. 2. The end date in dept_emp table is either NULL (meaning sometime future) or is in the past. 3. Let s assume the following statement has to be enforced: One employee can only be the manager of one department at a time. This implies that the date entries in dept_manager table cannot have overlaps for a certain employee. How would you ensure this? 6 Basketball Tournament Suppose that you were chosen to design a simple database for a basketball tournament. According to the project description, players belong to only one team and can be identified by their name. Each player has a number, and the total number of points scored by each player should be recorded. In this tournament, each team plays against each other. A team has exactly one coach and one captain. It has also a name and is identified by a subscription number. A coach has a unique name and an address. The captain is a real player while the coach is not. As soon as a match is done, the number of points scored by each team has to be stored separately. ER diagram of the tournament:

8 Relational model: coach: (name, ) player: (name, number, score, team_id) team: (sub_id, name, coach, captain) game: (teama, teamb, scorea, scoreb) 1. Create all required tables for your new database (using CREATE statements) including the appropriate primary and foreign key constraints. If you have cyclic foreign key dependencies (table A refers to table B and table B refers to table A), remember that you won t be able to insert any rows. You can always remove and add foreign keys using ALTER TABLE command before/after inserting rows. 2. After creating the tables, the project manager realizes that he forgot to give you some additional rules that the database must respect. Add these new constraints based on the following description (using ALTER TABLE statements). Do not forget to apply appropriate relational actions (e.g. CASCADE, NO ACTION, etc.) to foreign key constraints. Team names must be unique. Scores should always be non-negative. A game must be between different teams. Coaches cannot have the same address. Players have a number that range from four to 15. If a player is expelled from the tournament, he is discarded from the database. However, there must always be a captain. If the coach is expelled, its team is removed from the tournament but all its previous games should be kept (to compute final score of other teams). 3. After the tournament, the total number of points that each team scored during the whole tournament have to be computed. Additionally, in order to reward the best players, one would like to retrieve all players that score more than the half of the points of their team. For example, if a team has scored 100 points, we want to find the player that scored more than 50 points (if any). Write two SQL queries that express these requirements.