Department of Computer Science and Engineering, HKUST Slide 1 Notation Summary R, S: table schemas such as Branch (branch-name, branch-city, assets) Account (branch-name, account-number, balance) r 1,, r 2 : table instances (i.e., sets of tuples) r 1 (R 1 ): a table instance of table schema R 1 ; e.g., r 1 (Branch) is a set of branches t 1,, t 2 : tuples A, B, C, K 1, , etc., : a set of attributes; e.g., A could represent branch- name; in the slides, we use K 1, K 2, for primary keys and  for foreign key t 1 [A]: the projection of t 1 on attribute A If r 1 is an instance of Branch and t 1 is a tupe in r 1 then t 1 [branch-name] could be “Perryridge”  K (r 1 ): the set of values under column K of table instance r 1 (note: K could consist of more than one attribute/column)

Department of Computer Science and Engineering, HKUST Slide 2 Example of Mapping ISA into Tables Employee EmpNo Name Big table: Employee(EmpNo, Name, type, payperhour, nohours, salary) PT-Emp payperhour FT-Emp salary ISA Small tables: Employee ( EmpNo, Name) PT-Emp ( EmpNo, payperhour, nohours ) FT-Emp ( EmpNo, salary ) nohours Discussion: Problem with using a big table Is it necessary to add “type” in Employee? Is it convenient to add “type” in Employee? Given an EmpNo, how do you know if he/she a PT or FT employee? Can an employee be both FT and PT? How to restrict an employee to be either FT or PT? What are the foreign keys? Do you see a problem with the definition in Slide 6?

Department of Computer Science and Engineering, HKUST Slide 3 Discussion Problem with using a big table The table will have many columns with many null values in the table Is it necessary to add “type” in Employee? Not essential; to find out the type of an employee you can check if he/she exists in the PT-EMP or FT-EMP table Does it help to add “type” in Employee? Yes, checking both tables for the type of an employee is expensive Given an EmpNo, how do you know if he/she a PT or FT employee? See above; and try to write an SQL as an exercise Can an employee be both FT and PT? Yes if nothing is done How to restrict an employee to be either FT or PT? A constraint can be specified to ensure FT and PT tables are disjoint; there are more than one way to do it

Department of Computer Science and Engineering, HKUST Slide 4 Discussion What are the foreign keys? Do you see a problem with the definition in Slide 6? EmpNo is the foreign key in PT-EMP and FT-EMP tables Yes, because on Slide 6, the definition says a foreign key in a table cannot be the primary key of that table Finally, why is Employee needed? Can we get rid of it by putting Name in the PT-Emp and FT-Emp tables? Think about it yourselves