CMSC 424 Database design Lecture 4: Relational Model ER to Relational model. Book: Chap. 2 and 6. Mihai Pop

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1 CMSC 424 Database design Lecture 4: Relational Model ER to Relational model Book: Chap. 2 and 6 Mihai Pop

2 Oracle accounts SQL homework next week Administrative issues

3 Summary Entity-relationship Model Intuitive diagram-based representation of domain knowledge, data properties etc Two key concepts: Entities Relationships We also looked at: Relationship cardinalities Keys Weak entity sets

4 Summary Details unimportant Key idea: We can represent many data properties and constraints conceptually using this Read Chapter 6 Assignment will require you to do this anyway!

5 Relational Data Model Introduced by Ted Codd (late 60 s early 70 s Before = Network Data Model (Cobol as DDL, DML Very contentious: Database Wars (Charlie Bachman vs. Mike Stonebraker Relational data model contributes: Separation of logical, physical data models (data independence Declarative query languages Formal semantics Query optimization (key to commercial success 1 st prototypes: Ingres CA Postgres Illustra Informix IBM System R Oracle, DB2

6 Mathematical relations Why Called Relations? Given sets: R = {1, 2, 3}, S = {3, 4} R S = { (1, 3, (1, 4, (2, 3, (2, 4, (3, 3, (3, 4 } A relation on R, S is any subset ( of R S (e.g: { (1, 4, (3, 4} Database relations Given attribute domains Branches = { Downtown, Brighton, } Accounts = { A-101, A-201, A-217, } Balances = R Account Branches Accounts Balances { (Downtown, A-101, 500, (Brighton, A-201, 900, (Brighton, A-217, 500 }

7 Relations bname acct_no balance Account = Downtown Brighton Brighton A-101 A-201 A Considered equivalent to { (Downtown, A-101, 500, (Brighton, A-201, 900, (Brighton, A-217, 500 } Relational database semantics defined in terms of mathematical relations

8 1. Tables = Relations 2. Columns = Attributes 3. Rows = Tuples Terms and Definitions 4. Relation Schema (or Schema 1. A list of attributes and their domains 2. We will require the domains to be atomic 3. E.g. account(account-number, branch-name, balance 5. Relation Instance 1. A particular instantiation of a relation with actual values 2. Will change with time

9 So That s the basic relational model That s it? What about the constraints? How do we represent one-to-one vs many-to-one relationships? Many of those constraints get embedded in the schema Especially relationship cardinality constraints Others are explicitly represented using other constructs

10 E/R Diagrams Relations Convert entity sets into a relational schema with the same set of attributes Customer Customer_Schema(cname, ccity, cstreet cname ccity cstreet bname bcity assets Branch Branch_Schema(bname, bcity, assets

11 E/R Diagrams Relations Say One-to-Many Relationship from Customer to Account Many accounts per customer acct-no Account balance access-date Account_Schema(acct-no, balance, cname, access-date Depositor Customer Customer_Schema(cname, ccity, cstreet cname ccity cstreet

12 E/R Diagrams Relations Entity Sets E/R Relational Schema E,, a n a 1 a n

13 E/R Diagrams Relations Entity Sets E/R Relational Schema E,, a n a 1 a n Relationship Sets R, b 1, c 1,, c n R a 1 a n c 1 c k b 1 b m a 1 : s key b 1 : s key c 1,, c k : attributes of R Not the whole story for Relationship Sets

14 E/R Diagrams Relations Relationship Cardinality Relational Schema R a 1 a n c1 ck b 1 b m n:m R,, a n = (b 1,, b m R, b 1, c 1,, c n

15 E/R Diagrams Relations Relationship Cardinality Relational Schema R a 1 a n c1 ck b 1 b m n:m n:1 R R,, a n = (b 1,, b m R, b 1, c 1,, c n,, a n, b 1, c 1,, c n = (b 1,, b m

16 E/R Diagrams Relations Relationship Cardinality Relational Schema R a 1 a n c1 ck b 1 b m n:m n:1 1:n R R R,, a n = (b 1,, b m R, b 1, c 1,, c n,, a n, b 1, c 1,, c n = (b 1,, b m,, a n = (b 1,, b m,, a 1, c 1,, c n

17 E/R Diagrams Relations Relationship Cardinality Relational Schema R a 1 a n c1 ck b 1 b m n:m n:1 1:n 1:1 R R R R,, a n = (b 1,, b m R, b 1, c 1,, c n,, a n, b 1, c 1,, c n = (b 1,, b m,, a n = (b 1,, b m,, a 1, c 1,, c n Treat as n:1 or 1:n

18 Translating E/R Diagrams to Relations acct_no balance bname bcity assets Account Acct-Branch Branch Depositor Loan-Branch Customer Borrower Loan cname ccity cstreet lno amt Q. How many tables does this get translated into? A. 6 (account, branch, customer, loan, depositor, borrower

19 Bank Database A-101 A-215 A-102 A-305 A-201 A-222 A-217 Downtown Mianus Perry R.H. Brighton Redwood Brighton balance acct_no bname Account A-101 A-215 A-102 A-305 A-201 A-217 A-222 Johnson Smith Hayes Turner Johnson Jones Lindsay acct_no cname Depositor Harrison Rye Harrison Rye Pittsfield Stanford Princeton Pittsfield Palo Alto Woodside Brooklyn Stanford Main North Main North Park Putnam Nassau Spring Alma Sand Hill Senator Walnut Jones Smith Hayes Curry Lindsay Turner Williams Adams Johnson Glenn Brooks Green ccity cstreet cname Customer 9M 2.1M 1.7M 0.4M 8M 0.3M 3.7M 7.1M Brooklyn Palo Alto Horseneck Horseneck Horseneck Bennington Rye Brooklyn Downtown Redwood Perry Mianus R.H. Pownel N. Town Brighton assets bcity bname Branch L-17 L-23 L-15 L-14 L-93 L-11 L-17 L-16 Jones Smith Hayes Jackson Curry Smith Williams Adams lno cname Borrower L-17 L-23 L-15 L-14 L-93 L-11 L-16 Downtown Redwood Perry Downtown Mianus R.H. Perry amt lno bname Loan

20 E/R Diagrams & Relations Weak Entity Sets E/R IR Relational Schema,, a n, b 1,, b m a 1 a b n 1 b m

21 E/R Diagrams & Relations E/R Relational Schema Multivalued Attributes Employee Emp Emp-Phones = (ssn, name = (ssn, phone ssn name phone ssn 001 Smith Emp name ssn phone Emp-Phones

22 E/R Diagrams & Relations E/R Relational Schema Subclasses a 1 a n Method 1: E,, a n E, b 1,, b m, c 1,, c k ISA b b m 1 c 1 c k