M.P. Johnson, DBMS, Stern/NYU, Sp20041 C : Database Management Systems Lecture #10 Matthew P. Johnson Stern School of Business, NYU Spring, 2004

M.P. Johnson, DBMS, Stern/NYU, Sp Agenda Last time: R.A., Bags This time: 1. Finish R.A. 2. Begin SQL Project Part 2 due now Something else assigned soon

M.P. Johnson, DBMS, Stern/NYU, Sp Relational Algebra Review Five basic operators:  Union:  Intersection: Difference: -  Selection:   Projection:   Cartesian Product:  Extended operators:  Joins (equijoin, theta join, semijoin, outerjoin)  Renaming:   Extended projection   Sorting   Grouping-and-aggregation op 

M.P. Johnson, DBMS, Stern/NYU, Sp Sorting So far, everything’s an unordered bag But sometimes order is nice Sort op  L (R) produces a list, not a bag No operators operate on lists   if sort called, generally last op Subscript L = a 1,a 2,… in op is the list of attributes to sort on  Rows sorted by attributes  Rows with same a 1 value sorted by a 2, etc.

M.P. Johnson, DBMS, Stern/NYU, Sp Outerjoin Like L ⋈ R except that dangling tuples are included, padded with nulls Left outerjoin: dangling tuples from L are include  Nulls appear “on the right” Right outerjoin: dangling tuples from R are included  Nulls appear “on the left”

M.P. Johnson, DBMS, Stern/NYU, Sp Constraints on Relations (5.5) Ref. integ., FDs, other constraints are expressible in RA Two basic tools: 1. R =   Assert R is empty 2. R  S  Assert R is a subset of S NB: They’re equivalent R  S iff R – S =  R =  iff R  S-S (for arbitrary S)

M.P. Johnson, DBMS, Stern/NYU, Sp Expressing referential integrity Relations:  Reps(ssn, name, etc.)  Clients(ssn, name, rssn) Suppose we require: each client gets a sales rep a client’s row contains an rssn  must have a rep with that ssn How to require this in RA? Every Clients.rssn must be in the set of Reps.ssns  rssn (Clients)   ssn (Reps) Or:  rssn (Clients) –  ssn (Reps) = 

M.P. Johnson, DBMS, Stern/NYU, Sp Expressing referential integrity Also works for multiple attributes Relations: StarsIn(SName,Title,Year) Movies(Title, Year, Length, Studio) Require: every movie referenced by StarsIn to exist Write:  Title,Year (StarsIn)   Title,Year (Movies)

M.P. Johnson, DBMS, Stern/NYU, Sp Expressing FDs Relation: Employees(name,ssn,address,gender, etc.) Has FD: ssn  address What does the FD mean? No matter how we choose two rows, if they agree on ssn, then they agree on address So, strategy: choose pairs all possible ways; Select pairs that agree on ssn but not address; Check how many we get First, rename one copy to E1 and one to E2   E1 (Employees),  E2 (Employees) Then:  E1.ssn=E2.ssn AND E1.address != E2.address (E1 x E2) = 

M.P. Johnson, DBMS, Stern/NYU, Sp Expressing domain constraints Constraint on legal values for attributes Employees(name,ssn,address,gender etc.) Gender should be M/F Select bad ones and check count  gender!=‘F’ AND gender!=‘M’ (Employees) = 

M.P. Johnson, DBMS, Stern/NYU, Sp Expressing other constraints Relations: MovieExecs(name, address, ssn, netWorth) Studios(name, address, presSsn) Constraint: Studio presidents must be worth at leat $10,000,000 First, theta-join presSsn to ssn, then select ones w/ < $10M, then check count:  netWorth< (Studio ⋈ presSsn=ssn MEs) =  Or: Select MEs w/ >= $10M, then check that they contain all studio presidents:  presSsn (Studios)  ssn (  netWorth< (MEs))

M.P. Johnson, DBMS, Stern/NYU, Sp Recap: You are here First part of course is done: conceptual foundations You now know:  E/R Model  Relational Model  Relational Algebra You now know how to:  Capture part of world as an E/R model  Convert E/R models to relational models  Convert relational models to good (normal) forms  Express queries in relational algebra Next:  Create, update, query SQL tables  Write SQL/DB-connected applications

M.P. Johnson, DBMS, Stern/NYU, Sp Next topic: SQL (6.1) Standard language for querying and manipulating data Structured Query Language Many standards: ANSI SQL, SQL92/SQL2, SQL3/SQL99 Vendors support various subsets/extensions We’ll do SQL99/Oracle Basic form (many more bells and whistles in addition): SELECT attributes FROM relations (possibly multiple, joined) WHERE conditions (selections) SELECT attributes FROM relations (possibly multiple, joined) WHERE conditions (selections)

M.P. Johnson, DBMS, Stern/NYU, Sp Data Types in SQL Characters:  CHAR(20)-- fixed length  VARCHAR(40)-- variable length Numbers:  BIGINT, INT, SMALLINT, TINYINT  REAL, FLOAT -- differ in precision  MONEY Times and dates:  DATE  DATETIME-- SQL Server

M.P. Johnson, DBMS, Stern/NYU, Sp “Tables” PNamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks SingleTouch$149.99PhotographyCanon MultiTouch$203.99HouseholdHitachi Product Attribute names Table name Tuples or rows

M.P. Johnson, DBMS, Stern/NYU, Sp Simple SQL Query PNamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks SingleTouch$149.99PhotographyCanon MultiTouch$203.99HouseholdHitachi SELECT * FROM Product WHERE category=‘Gadgets’ Product PNamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks “selection”

M.P. Johnson, DBMS, Stern/NYU, Sp Simple SQL Query PNamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks SingleTouch$149.99PhotographyCanon MultiTouch$203.99HouseholdHitachi SELECT PName, Price, Manufacturer FROM Product WHERE Price > 100 Product PNamePriceManufacturer SingleTouch$149.99Canon MultiTouch$203.99Hitachi “selection” and “projection”

M.P. Johnson, DBMS, Stern/NYU, Sp A Notation for SQL Queries SELECT Name, Price, Manufacturer FROM Product WHERE Price > 100 Product(PName, Price, Category, Manfacturer) Answer(PName, Price, Manfacturer) Input Schema Output Schema

M.P. Johnson, DBMS, Stern/NYU, Sp R.A.  SQL R.A. Projection   SQL SELECT R.A. Selection   SQL WHERE R.A. Join  SQL FROM  Comma-separated list… What goes in the WHERE clause: x = y, x < y, x <= y, etc.  For number, they have the usual meanings  For CHAR and VARCHAR: lexicographic ordering Expected conversion between CHAR and VARCHAR  For dates and times, what you expect

M.P. Johnson, DBMS, Stern/NYU, Sp R.A.  SQL Movies(Title,Year,Length,inColor,Studio,Prdcr#) Q: How long was Star Wars (1977), in R.A.? Q: In SQL? Q: Which Fox movies are are at least 100 minutes long, in R.A.? Q: In SQL?

M.P. Johnson, DBMS, Stern/NYU, Sp R.A.  SQL Reps(ssn, name, etc.) Clients(ssn, name, rssn) Q: Who are George’s clients, in R.A.? Second answer from last time:   Clients.name (  Reps.name=“George” and Reps.ssn=rssn (Reps x Clients)) In SQL?

M.P. Johnson, DBMS, Stern/NYU, Sp The LIKE operator s LIKE p: pattern matching on strings p may contain two special symbols:  _ = any single character  % = zero or more chars Product(Name, Price, Category, Manufacturer) Find all products whose name contains ‘gizmo’: SELECT * FROM Products WHERE PName LIKE ‘%gizmo%’

M.P. Johnson, DBMS, Stern/NYU, Sp The LIKE operator Q: What it want to search for values containing a ‘%’? PName LIKE ‘%%’ won’t work Instead, must use escape chars In C/C++/J, prepend ‘\’ In SQL, prepend an arbitrary escape char: PName LIKE ‘x%x%’ ESCAPE ‘x’

M.P. Johnson, DBMS, Stern/NYU, Sp Eliminating Duplicates SELECT DISTINCT category FROM Product SELECT DISTINCT category FROM Product Compare to: SELECT category FROM Product SELECT category FROM Product Category Gadgets Photography Household Category Gadgets Photography Household

M.P. Johnson, DBMS, Stern/NYU, Sp Ordering the Results Ordering is ascending, unless you specify the DESC keyword per attribute. SELECT pname, price, manufacturer FROM Product WHERE category=‘gizmo’ AND price > 50 ORDER BY price, pname SELECT pname, price, manufacturer FROM Product WHERE category=‘gizmo’ AND price > 50 ORDER BY price, pname SELECT pname, price, manufacturer FROM Product WHERE category=‘gizmo’ AND price > 50 ORDER BY price DESC, pname ASC SELECT pname, price, manufacturer FROM Product WHERE category=‘gizmo’ AND price > 50 ORDER BY price DESC, pname ASC

M.P. Johnson, DBMS, Stern/NYU, Sp Ordering the Results SELECTCategory FROMProduct ORDER BYPName SELECTCategory FROMProduct ORDER BYPName PNamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks SingleTouch$149.99PhotographyCanon MultiTouch$203.99HouseholdHitachi ?

M.P. Johnson, DBMS, Stern/NYU, Sp Ordering the Results SELECT DISTINCT category FROMProduct ORDER BYcategory SELECT DISTINCT category FROMProduct ORDER BYcategory Compare to: Category Gadgets Household Photography SELECT DISTINCT category FROMProduct ORDER BYPName SELECT DISTINCT category FROMProduct ORDER BYPName ?

M.P. Johnson, DBMS, Stern/NYU, Sp Joins in SQL (6.2) Connect two or more tables: PNamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks SingleTouch$149.99PhotographyCanon MultiTouch$203.99HouseholdHitachi Product Company CNameStockPriceCountry GizmoWorks25USA Canon65Japan Hitachi15Japan What is the connection between them?

M.P. Johnson, DBMS, Stern/NYU, Sp Joins in SQL Product (pname, price, category, manufacturer) Company (cname, stockPrice, country) Find all products under $200 manufactured in Japan; return their names and prices. SELECT PName, Price FROM Product, Company WHERE Manufacturer=CName AND Country=‘Japan’ AND Price <= 200 Join between Product and Company

M.P. Johnson, DBMS, Stern/NYU, Sp Joins in SQL PNamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks SingleTouch$149.99PhotographyCanon MultiTouch$203.99HouseholdHitachi Product Company CnameStockPriceCountry GizmoWorks25USA Canon65Japan Hitachi15Japan PNamePrice SingleTouch$ SELECT PName, Price FROM Product, Company WHERE Manufacturer=CName AND Country=‘Japan’ AND Price <= 200

M.P. Johnson, DBMS, Stern/NYU, Sp Joins in SQL Product (pname, price, category, manufacturer) Company (cname, stockPrice, country) Find all countries that manufacture some product in the ‘Gadgets’ category. SELECTCountry FROMProduct, Company WHEREManufacturer=CName AND Category=‘Gadgets’

M.P. Johnson, DBMS, Stern/NYU, Sp Joins in SQL NamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks SingleTouch$149.99PhotographyCanon MultiTouch$203.99HouseholdHitachi Product Company CnameStockPriceCountry GizmoWorks25USA Canon65Japan Hitachi15Japan Country ?? What is the problem? What’s the solution? SELECT Country FROM Product, Company WHERE Manufacturer=CName AND Category=‘Gadgets’

M.P. Johnson, DBMS, Stern/NYU, Sp Joins Product (pname, price, category, manufacturer) Purchase (buyer, seller, store, product) Person(name, phone, city) Find names of Seattleites who bought Gadgets, and the names of the stores they bought such product from. SELECT DISTINCT name, store FROM Person, Purchase, Product WHERE persname=buyer AND product = pname AND city=‘Seattle’ AND category=‘Gadgets’

M.P. Johnson, DBMS, Stern/NYU, Sp Disambiguating Attributes Sometimes two relations have the same attr: Person(pname, address, worksfor) Company(cname, address) SELECT DISTINCT pname, address FROM Person, Company WHERE worksfor = cname SELECT DISTINCT Person.pname, Company.address FROM Person, Company WHERE Person.worksfor = Company.cname Which address ?

M.P. Johnson, DBMS, Stern/NYU, Sp Tuple Variables SELECT DISTINCT x.store FROM Purchase AS x, Purchase AS y WHERE x.product = y.product AND y.store = ‘BestBuy’ SELECT DISTINCT x.store FROM Purchase AS x, Purchase AS y WHERE x.product = y.product AND y.store = ‘BestBuy’ Find all stores that sold at least one product that the store ‘BestBuy’ also sold: Answer (store) Product (pname, price, category, manufacturer) Purchase (buyer, seller, store, product) Person(persname, phoneNumber, city)

M.P. Johnson, DBMS, Stern/NYU, Sp Tuple Variables Tuple variables introduced automatically: Product ( name, price, category, manufacturer) Becomes: Doesn’t work when Product occurs more than once In that case the user needs to define variables explicitly SELECT name FROM Product WHERE price > 100 SELECT name FROM Product WHERE price > 100 SELECT Product.name FROM Product AS Product WHERE Product.price > 100 SELECT Product.name FROM Product AS Product WHERE Product.price > 100

M.P. Johnson, DBMS, Stern/NYU, Sp SQL Query Semantics SELECT a1, a2, …, ak FROM R1 AS x1, R2 AS x2, …, Rn AS xn WHERE Conditions 1. Nested loops: Answer = {} for x1 in R1 do for x2 in R2 do ….. for xn in Rn do if Conditions then Answer = Answer  {(a1,…,ak)} return Answer Answer = {} for x1 in R1 do for x2 in R2 do ….. for xn in Rn do if Conditions then Answer = Answer  {(a1,…,ak)} return Answer

M.P. Johnson, DBMS, Stern/NYU, Sp SQL Query Semantics SELECT a1, a2, …, ak FROM R1 AS x1, R2 AS x2, …, Rn AS xn WHERE Conditions 2. Parallel assignment Doesn’t impose any order! Answer = {} for all assignments x1 in R1, …, xn in Rn do if Conditions then Answer = Answer  {(a1,…,ak)} return Answer Answer = {} for all assignments x1 in R1, …, xn in Rn do if Conditions then Answer = Answer  {(a1,…,ak)} return Answer

M.P. Johnson, DBMS, Stern/NYU, Sp First Unintuitive SQLism SELECTR.A FROMR, S, T WHERER.A=S.A OR R.A=T.A Looking for R  (S  T) But what happens if T is empty? See transcript of this in Oracle on salestranscript