1. COMP 430 Intro. to Database Systems ER Implementation as Tables Slides use ideas from Chris Ré.

2. Book Writes Publishes GoesOnLoan Borrower Author ID address A Solution to Library Activity Publisher Loan Borrows name ISBN title num_copies address name card_num date Chapter PartOf title ID Writing  is_self_pub

3. ER implementation overview Author IDname Author IDname …… product_idstate_iddealer_id ……… Concession Except, sometimes optimize away this table: Common cases of one-to-many One-to-one Concession Dealer Product State Primary keys = keys of entity sets on a “many” side of relation.

4. Arity & partiticipation: Common cases

5. Many-to-many Product IDname …… Person IDname …… CREATE TABLE Product ( … PRIMARY KEY (ID) ); CREATE TABLE Person ( … PRIMARY KEY (ID) ); Purchase ProductPerson name ID name ID Purchase prod_IDperson_ID …… CREATE TABLE Purchase ( … PRIMARY KEY (prod_ID, person_ID), FOREIGN KEY (prod_ID) REFERENCES Product (ID), FOREIGN KEY (person_ID) REFERENCES Person (ID) );

6. Many-to-many Purchase ProductPerson name ID name ID Purchase prod_IDperson_ID …… CREATE TABLE Purchase ( … PRIMARY KEY (prod_ID, person_ID), FOREIGN KEY (prod_ID) REFERENCES Product (ID), FOREIGN KEY (person_ID) REFERENCES Person (ID) ); Primary key fields also NOT NULL. Doesn’t that force total participation? No. E.g., a Product never purchased simply never appears in Purchase table.

7. N-ary many-many Purchase Product Person Store Junction table’s primary key = combination of primary keys of all n tables.

8. One-to-many / many-to-one Book Publishes IDaddress Publisher name title num_copies is_self_pub ISBN Publisher IDnameaddress ……… Book ISBNtitlenum_copiesis_self_pubpub_IDpub_date …………… CREATE TABLE Publisher ( … PRIMARY KEY (ID) ); CREATE TABLE Book ( … PRIMARY KEY (ISBN), FOREIGN KEY (pub_ID) REFERENCES Publisher (ID) ); date Publishes table optimized away.

9. One-to-many / many-to-one Book Publishes IDaddress Publisher name title num_copies is_self_pub ISBN Publisher IDnameaddress ……… Way to remember where the additional field is placed: Book has one publisher, so it can be a field. Publisher has many books, so it can’t be a field. date Book ISBNtitlenum_copiesis_self_pubpub_IDpub_date ……………

10. One-to-many with total participation Product Makes IDaddress Company name ID Company IDnameaddress ……… Product IDnameco_ID ……… CREATE TABLE Company ( … PRIMARY KEY (ID) ); CREATE TABLE Product ( … co_ID VARCHAR(50) NOT NULL, PRIMARY KEY (ID), FOREIGN KEY (co_ID) REFERENCES Company (ID) ); Related item must have a value.

11. One-to-many with weak entity Team Affiliation University namesport ID University name … Team univ_nameIDsport ……… CREATE TABLE University ( … PRIMARY KEY (name) ); CREATE TABLE Team ( … PRIMARY KEY (univ_name, ID), FOREIGN KEY (univ_name) REFERENCES University (name) ); Related item must have a value.

12. Activity: Implement three similar ideas Product name category price Person p_id Purchased date Product name category price Person p_id date Purchase quantitypurch_id ProductOfBuyerOf Product name category price Person p_id date Purchase quantity ProductOfBuyerOf Each results in a Purchase junction table. But what primary keys does it have?

13. One-to-many with total participation Concession p_idstate_idd_iddate ………… CREATE TABLE Concession ( … PRIMARY KEY (p_id, state_id), FOREIGN KEY (p_id) REFERENCES Product (p_id), FOREIGN KEY (state_id) REFERENCES State (state_id), FOREIGN KEY (d_id) REFERENCES Dealer (d_id) ); Concession Dealer Product State p_id state_id d_id date

14. One-to-one with same key Player ContactInfoStatistics name IDpointsIDaddress Player IDnameaddresspoints ………… Usually indicates a poor design. Combine into one entity set.

15. One-to-one with different keys Chair FacultyDept name emp_IDnamedept_IDoffice Faculty emp_IDnameofficedept_id ………… Sometimes indicates a poor design. Dept dept_IDnamechair_id ……… CREATE TABLE Faculty ( … PRIMARY KEY (emp_ID), FOREIGN KEY (dept_ID) REFERENCES Dept (dept_ID) ); CREATE TABLE Department ( … PRIMARY KEY (dept_ID), FOREIGN KEY (chair_ID) REFERENCES Faculty (emp_ID) ); Same optimization as for one-to-many.

16. Parallel one-to-many/one relationships Vehicle Owns IDaddress Person name modelID Services Person IDnameaddress ……… Vehicle IDmodelowner_IDmechanic_ID ……… CREATE TABLE Person ( … PRIMARY KEY (ID) ); CREATE TABLE Vehicle ( … PRIMARY KEY (ID), FOREIGN KEY (owner_ID) REFERENCES Person (owner_ID), FOREIGN KEY (mechanic_ID) REFERENCES Person (owner_ID) ); Same idea as before, for each relationship.

17. Arity & partiticipation: Some less common cases

18. Many-to-many with total participation Purchase ProductPerson name ID name ID Purchase prod_IDperson_ID …… CREATE TABLE Purchase ( … PRIMARY KEY (prod_ID, person_ID), FOREIGN KEY (prod_ID) REFERENCES Product (ID), FOREIGN KEY (person_ID) REFERENCES Person (ID) ); Can enforce with a CHECK constraint. See later.

19. Many-to-many with total participation Purchase ProductPerson name ID name ID Classic chicken-and-egg problem: No Product can be created without a related Person. No Person can be created without a related Product.

20. One-to-many with total participation Product Makes IDaddress Company name ID Company IDnameaddress ……… Product IDname …… CREATE TABLE Company ( … PRIMARY KEY (ID) ); CREATE TABLE Product ( … PRIMARY KEY (ID) ); Makes co_IDprod_ID …… CREATE TABLE Makes ( … PRIMARY KEY (co_ID), FOREIGN KEY (co_ID) REFERENCES Company (ID), FOREIGN KEY (prod_ID) REFERENCES Product (ID) ); Can use CHECK constraint.

21. Subclasses, superclasess, unions

22. Subclasses – Three strategies Product name price Software Product platforms Educational Product age_group   Dealing with sub- & superclasses is one of relational database’s weaknesses.

23. Subclasses – Strategy 1 Product name price Software Product platforms Educational Product age_group   Product(name, price) SoftwareProduct(name, platforms) EducationalProduct(name, age_group) + Semantically, most accurate representation. - Typically leads to lots of joins between sub- & superclass

24. Subclasses – Strategy 2 HourlyEmployee(ID, name, rate) SalaryEmployee(ID, name, salary) + Avoids joins between sub- & superclasses. -Requires disjoint subclasses. -Poor if superclass is directly related to other entity sets. Employee ID name HourlyEmployee rate SalaryEmployee salary D  

25. Subclasses – Strategy 3 Product name price Software Product platforms Educational Product age_group   Product(name, price, platforms, age_group) Use NULL when attribute not applicable. + Avoids joins between sub- & superclasses. - Joins between subclass & other entity sets now involve more data.

26. Book Writes Publishes GoesOnLoan Borrower Author ID address Activity: Implemen t as Schemas Publisher Loan Borrows name ISBN title num_copies address name card_num date Chapter PartOf title ID Writing  is_self_pub