CMSC424: Database Design Instructor: Amol Deshpande

Today… Integrity Constraints Relational Database Design

Referential Integrity Constraints Idea: prevent “dangling tuples” (e.g.: a loan with a bname, Kenmore, when no Kenmore tuple in branch) Referencing Relation (e.g. loan) Referenced Relation (e.g. branch) “foreign key” bname primary key bname Ref Integrity: ensure that: foreign key value  primary key value (note: don’t need to ensure , i.e., not all branches have to have loans)

Referential Integrity Constraints Referencing Relation (e.g. loan) Referenced Relation (e.g. branch) bname x x x In SQL: CREATE TABLE branch( bname CHAR(15) PRIMARY KEY....) CREATE TABLE loan ( FOREIGN KEY bname REFERENCES branch); Affects: 1) Insertions, updates of referencing relation 2) Deletions, updates of referenced relation

Referential Integrity Constraints c c x x x A B what happens when we try to delete this tuple? titi tjtj Ans: 3 possibilities 1) reject deletion/ update 2) set t i [c], t j [c] = NULL 3) propagate deletion/update DELETE: delete ti, tj UPDATE: set ti[c], tj[c] to updated values

Referential Integrity Constraints c c x x x A B what happens when we try to delete this tuple? titi tjtj CREATE TABLE A (..... FOREIGN KEY c REFERENCES B action ) Action: 1) left blank (deletion/update rejected) 2) ON DELETE SET NULL/ ON UPDATE SET NULL sets ti[c] = NULL, tj[c] = NULL 3) ON DELETE CASCADE deletes ti, tj ON UPDATE CASCADE sets ti[c], tj[c] to new key values

Global Constraints Idea: two kinds 1) single relation (constraints spans multiple columns) E.g.: CHECK (total = svngs + check) declared in the CREATE TABLE 2) multiple relations: CREATE ASSERTION SQL examples: 1) single relation: All Bkln branches must have assets > 5M CREATE TABLE branch ( bcity CHAR(15), assets INT, CHECK (NOT(bcity = ‘Bkln’) OR assets > 5M)) Affects: insertions into branch updates of bcity or assets in branch

Global Constraints SQL example: 2) Multiple relations: every loan has a borrower with a savings account CHECK (NOT EXISTS ( SELECT * FROM loan AS L WHERE NOT EXISTS( SELECT * FROM borrower B, depositor D, account A WHERE B.cname = D.cname AND D.acct_no = A.acct_no AND L.lno = B.lno))) Problem: Where to put this constraint? At depositor? Loan?.... Ans: None of the above: CREATE ASSERTION loan-constraint CHECK(..... ) Checked with EVERY DB update! very expensive.....

Summary: Integrity Constraints Constraint TypeWhere declaredAffects...Expense Key Constraints CREATE TABLE (PRIMARY KEY, UNIQUE) Insertions, UpdatesModerate Attribute Constraints CREATE TABLE CREATE DOMAIN (Not NULL, CHECK) Insertions, UpdatesCheap Referential Integrity Table Tag (FOREIGN KEY.... REFERENCES....) 1.Insertions into referencing rel’n 2. Updates of referencing rel’n of relevant attrs 3. Deletions from referenced rel’n 4. Update of referenced rel’n 1,2: like key constraints. Another reason to index/sort on the primary keys 3,4: depends on a. update/delete policy chosen b. existence of indexes on foreign key Global ConstraintsTable Tag (CHECK) or outside table (CREATE ASSERTION) 1. For single rel’n constraint, with insertion, deletion of relevant attrs 2. For assesrtions w/ every db modification 1. cheap 2. very expensive

SQL Is that it ? Unfortunately No SQL 3 standard is several hundreds of pages (if not several thousands) And expensive too.. We will discuss a few more constructs along the way E.g. Embedded SQL, creating indexes etc Again, this is what the reference books are for; you just need to know where to look in the reference book

Questions ? Next: Relational Database Design

Where did we come up with the schema that we used ? E.g. why not store the actor names with movies ? Topics: Formal definition of what it means to be a “good” schema. How to achieve it.

Movies Database Schema Movie(title, year, length, inColor, studioName, producerC#) StarsIn(movieTitle, movieYear, starName) MovieStar(name, address, gender, birthdate) MovieExec(name, address, cert#, netWorth) Studio(name, address, presC#) Movie(title, year, length, inColor, studioName, producerC#, starName) MovieStar(name, address, gender, birthdate) MovieExec(name, address, cert#, netWorth) Studio(name, address, presC#) Changed to:

TitleYearLengthinColorStudioNameprodC#StarName Star wars YesFox128Hamill Star wars YesFox128Fisher Star wars YesFox128H. Ford King Kong YesUniversal150Watts King Kong noRKO20Fay Issues: 1.Redundancy  higher storage, inconsistencies (“anomalies”) 2.Need nulls Unable to represent some information without using nulls How to store movies w/o actors (pre-productions etc) ? Movie(title, year, length, inColor, studioName, producerC#, starName)

TitleYearLengthinColorStudioNameprodC#StarNames Star wars YesFox128{Hamill, Fisher, H. ford} King Kong YesUniversal150Watts King Kong noRKO20Fay Issues: 3. Avoid sets - Hard to represent - Hard to query Movie(title, year, length, inColor, studioName, producerC#, starNames)

NameAddress FoxAddress1 Studio2Address1 UniversialAddress2 This process is also called “decomposition” Issues: 4. Requires more joins (w/o any obvious benefits) 5. Hard to check for some dependencies What if the “address” is actually the presC#’s address ? No easy way to ensure that constraint (w/o a join). Split Studio(name, address, presC#) into: Studio1 (name, presC#) Studio2(name, address)??? NamepresC# Fox101 Studio2101 Universial102 Smaller schemas always good ????

movieTitlestarName Star WarsHamill King KongWatts King KongFaye Issues: 6. “joining” them back results in more tuples than what we started with (King Kong, 1933, Watts) & (King Kong, 2005, Faye) This is a “lossy” decomposition We lost some constraints/information The previous example was a “lossless” decomposition. Decompose StarsIn(movieTitle, movieYear, starName) into: StarsIn1(movieTitle, movieYear) StarsIn2(movieTitle, starName) ??? movieTitlemovieYear Star wars1977 King Kong1933 King Kong2005 Smaller schemas always good ????

Desiredata No sets Correct and faithful to the original design Avoid lossy decompositions As little redundancy as possible To avoid potential anomalies No “inability to represent information” Nulls shouldn’t be required to store information Dependency preservation Should be possible to check for constraints

Approach We will encode and list all our knowledge about the schema somehow Functional dependencies (FDs) SSN  name (SSN “implies” length) If two tuples have the same “SSN”, they must have the same “name” movietitle  length --- Not true. But, (movietitle, movieYear)  length --- True. We will define a set of rules that the schema must follow to be considered good “Normal forms”: 1NF, 2NF, 3NF, BCNF, 4NF, … Rules specify constraints on the schemas and FDs