Principles of Database Systems CSE 544p. Lecture #1 September 28, 2011

Transcription

1 Principles of Database Systems CSE 544p Lecture #1 September 28,

2 Staff Instructor: Dan Suciu CSE 662, Office hours: Wednesdays, 5:30-6:20 TAs: Sandra Fan, 2

3 CommunicaRons Web page: hsp:// Lectures will be available here Homework will be posted here Announcements may be posted here Mailing list: Announcements, group discussions If you registered, you are automarcally subscribed 3

4 Textbook(s) Main textbook: Database Management Systems, Ramakrishnan and Gehrke Second textbook: Database Systems: The Complete Book, Garcia- Molina, Ullman, Widom 4

5 Course Format Lectures Wednesdays, 6:30-9:20 7 Homework Assignments Take- home Final 5

6 Grading Homework: 70 % Take- home Final: 30% 6

7 Homework Assignments 1. SQL 2. Conceptual design 3. JAVA/SQL 4. TransacRons 5. Database tuning 6. XML/XPath/XQuery 7. Pig LaRn, on AWS Due: Mondays, by 11:59pm. Three late days per person 7

8 Take- home Final Posted on December 8, at 11:59pm Due on December 10, by 10:00pm No late days/hours/minutes/seconds 8

9 Sohware Tools Postgres: Preferred usage: download from download hsp:// Other opron: use postgres on lab machines SQL Server 2008 Download client from hsp://msdnaa.cs.washington.edu Username is your address Doesn t work? ms- sw- admin@cs.washington.edu Connect to IPROJSRV (may need tunneling) OK to use you own server, just import IMDB Xquery: download one interpreter from Preferred: Saxon: hsp://saxon.sourceforge.net/ (from apache; very popular) Others: Zorba: hsp:// xquery.com/ (I used this one: ½ day installaron) Galax: hsp://galax.sourceforge.net/ (great in the past, seems less well maintained) Pig LaRn: We will run it on Amazon Web Services You may download from hsp://hadoop.apache.org/pig/, but you won t need it 9

10 Accessing SQL Server SQL Server Management Studio Server Type = Database Engine Server Name = IPROJSRV AuthenRcaRon = SQL Server AuthenRcaRon Login = your UW address (not the CSE ) Password = [in class] Must connect from within CSE, or must use tunneling AlternaRvely: install your own, get it from MSDNAA (see earlier slide) Then play with IMDB, start working on HW 1 10

11 Rest of Today s Lecture Overview of DBMS Overview of the course content SQL 11

12 Database What is a database? Give examples of databases 12

13 Database What is a database? A collecron of files storing related data Give examples of databases Accounts database; payroll database; UW s students database; Amazon s products database; airline reservaron database 13

14 Database Management System What is a DBMS? Give examples of DBMS 14

15 Database Management System What is a DBMS? A big C program wri;en by someone else that allows us to manage efficiently a large database and allows it to persist over long periods of Bme Give examples of DBMS DB2 (IBM), SQL Server (MS), Oracle, Sybase MySQL, Postgres, SQL for Nerds, Greenspun, hsp://philip.greenspun.com/sql/ (Chap 1) 15

16 Market Shares From 2006 Gartner report: IBM: 21% market with $3.2BN in sales Oracle: 47% market with $7.1BN in sales Microsoh: 17% market with $2.6BN in sales 16

17 An Example The Internet Movie Database hsp:// EnRRes: Actors (800k), Movies (400k), Directors, RelaRonships: who played where, who directed what, 17

18 Key concept 1: RelaRonal Data Model Actor: id fname lname gender Cast: pid mid Tom Hanks M Amy Hanks F Movie: id Name year Toy Story

19 Key concept 2: DeclaraRve Query SELECT * FROM Actor Language SQL SELECT * FROM Actor WHERE lname = Hanks We write what we want, not how we want it. SELECT count(*) FROM Actor 19

20 Key concept 3: Data Independence SELECT * FROM Actor, Casts, Movie WHERE lname='hanks' and Actor.id = Casts.pid and Casts.mid=Movie.id and Movie.year= k actors, 3.5M casts, 380k movies; How can it be so fast? Physical data independence: query is independent of physical storage 20

21 How Can We Evaluate the Query? Actor: Cast: Movie: id fname lname gender pid mid id Name year... Hanks Plan 1:.... [ in class ] Plan 2:.... [ in class ] 21

22 Indexes: on Actor.lName, on Movie.year AlternaRve query plans: σ lname= Hanks σ year=1995 σ lname= Hanks σ year=1995 Actor Cast Movie Actor Cast Movie Query oprmizaron Database StaRsRcs histograms, synopses, etc 22

23 Key concept 4: TransacRons Recovery from systems failures: Transfer $100 from account 1 to account 2: X = Read(Account_1); X.amount = X.amount - 100; Write(Account_1, X); Y = Read(Account_2); Y.amount = Y.amount + 100; Write(Account_2, Y); What is the problem? CRASH! 23

24 Concurrency Control Overdrahing an account: User 1: User 2: X = Read(Account); if (X.amount >= 100) { dispense_money( ); X.amount = X.amount 100; } else error( Insufficient funds ); X = Read(Account); if (X.amount >= 100) { dispense_money( ); X.amount = X.amount 100; } else error( Insufficient funds ); What can go wrong? 24

25 TransacRons ACID = Atomicity ( = recovery) Consistency IsolaRon ( = concurrency control) Durability 25

26 Client/Server Database Architecture Single server that stores the database Many clients running apps and connecrng to DBMS Performance boslenecks: Client/server communicaron TransacRonal semanrcs Other architectures: main memory database replicated databases 26

27 Two Types of Database Usage OLTP (online- transacron- processing) Many updates Many simple point queries Few (or no) complex aggregate queries Decision- Support Many aggregate/group- by queries. Few (or no) updates 27

28 Trends in Data Management Large scale data analyrcs: Map/Reduce, Pig, Cloud based database service: AWS, Azure, NoSQL: sacrifice ACID for performance Data privacy Data provenance Complex data analyrcs: probabilisrc databases 28

29 Outline of Course Content 1. SQL 2. RelaRonal Calculus, Database Design 3. Constraints, Views 4. TransacRons: recovery 5. TransacRons: concurrency control 6. XML, XPath, XQuery 7. Data storage, indexes, physical tuning 8. Query execuron 9. Query oprmizaron 10. Big Data: Parallel databases, Map/Reduce, Pig LaRn 11. Advanced topics: privacy, provenance, probabilisrc dbs 29

30 Announcement: Homework 1 Homework 1 is posted; Due on Monday, Oct. 10 Tools: Postgres: install on your computer (PREFERRED) or use the installaron in the lab SQL Server, for tesrng only; connect to IPROJSRV: login: your UW address; password:.. Tasks: create db, import data, create indices, write 11 SQL queries 30

31 Outline for rest of today Basics SQL (Chapters 5.2, 5.3) Aggregates (Chapter 5.5.) Nulls, Outer joins (Chapter 5.6) Subqueries (Chapters 5.4) This is tough! Next lecture we will discuss RelaRonal Calculus (a.k.a. Tuple Calculus, Chapter 4.3). See supplementary text Three Query Language Formalisms 31

32 SQL Data DefiniRon Language (DDL) Create/alter/delete tables and their asributes Read from the book Data ManipulaRon Language (DML) Query tables, Insert/delete/modify Discussed in class 32

33 Table name Tables in SQL Attribute names Product Key PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi Tuples or rows 33

34 The RelaRonal Data Model Data is stored in tables, a.k.a. relabons Each relaron has: 1. A schema = name+asributes Product(PName, Price, Category, Manufacturer) Each relaron has a key, which we underline 2. An instance = set of rows SQL departs from the pure relaronal model in that it allows duplicate tuples Set semanbcs à bag semanbcs {1, 2, 3} à {1, 1, 2, 3, 3, 3} 34

35 Data Types in SQL Atomic types: Characters: CHAR(20), VARCHAR(50) Numbers: INT, BIGINT, SMALLINT, FLOAT Others: MONEY, DATETIME, Record (aka tuple) Has atomic asributes Table (relaron) A set of tuples 35

36 Simple SQL Query Product PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi SELECT * FROM Product WHERE category= Gadgets selection PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks 36

37 Simple SQL Query Product PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi SELECT PName, Price, Manufacturer FROM Product WHERE Price > $100 selection and projection PName Price Manufacturer SingleTouch $ Canon MultiTouch $ Hitachi 37

38 Details Case insensirve: SELECT = Select = select Product = product BUT: SeaSle seasle Constants: abc - yes abc - no 38

39 EliminaRng Duplicates SELECT DISTINCT category FROM Product Category Gadgets Photography Household Compare to: SELECT category FROM Product Category Gadgets Gadgets Photography Household 39

40 Ordering the Results SELECT pname, price, manufacturer FROM Product WHERE category= Gadgets AND price > $10 ORDER BY price, pname Ties are broken by the second attribute on the ORDER BY list. Ordering is ascending, unless you specify the DESC keyword. 40

41 PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi SELECT DISTINCT category FROM Product ORDER BY category SELECT Category FROM Product ORDER BY PName?? SELECT DISTINCT category FROM Product ORDER BY PName? 41

42 Keys and Foreign Keys Company Key Product CName GizmoWorks Canon Hitachi Country USA Japan Japan PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi Foreign key 42

43 Joins Product (PName, Price, Category, Manufacturer) Company (CName,, Country) Find all products under $200 manufactured in Japan; return their names and prices. Join between Product and Company SELECT PName, Price FROM Product, Company WHERE Manufacturer=CName AND Country= Japan AND Price <= $200 43

44 Joins Product PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi Company Cname GizmoWorks Canon Hitachi Country USA Japan Japan SELECT PName, Price FROM Product, Company WHERE Manufacturer=CName AND Country= Japan AND Price <= $200 PName Price SingleTouch $

45 Tuple Variables Product (PName, Price, Category, Manufacturer) Company (CName,, Country) Person(name, Country, Worksfor) SELECT DISTINCT name, country FROM Person, Company WHERE worksfor = cname Which country? SELECT DISTINCT Person.name, Company.country FROM Person, Company WHERE Person.worksfor = Company.cname SELECT DISTINCT x.name, y.country FROM Person AS x, Company AS y WHERE x.worksfor Dan Suciu = y.cname -- p544 Fall

46 In Class Product (pname, price, category, manufacturer) Company (cname, country) Find all Chinese companies that manufacture products in the toy category SELECT cname FROM WHERE 46

47 In Class Product (pname, price, category, manufacturer) Company (cname, country) Find all Chinese companies that manufacture products both in the electronic and toy categories SELECT cname FROM WHERE 47

48 The Nested Loop SemanRcs of SQL Queries SELECT a 1, a 2,, a k FROM R 1 AS x 1, R 2 AS x 2,, R n AS x n WHERE CondiRons Answer = {} for x 1 in R 1 do for x 2 in R 2 do.. for x n in R n do if Conditions then Answer = Answer {(a 1,,a k )} return Answer 48

49 Using the Formal SemanRcs What do these queries compute? SELECT DISTINCT R.A FROM R, S WHERE R.A=S.A Returns R S SELECT DISTINCT R.A FROM R, S, T WHERE R.A=S.A OR R.A=T.A If S and T then returns R (S T) else returns 49

50 AggregaRon Product (pname, price, category, manufacturer) Company (cname, country) SELECT count(*) FROM Product SELECT sum(price) FROM Product WHERE manufacturer= GizmoWorks SQL supports several aggregation operations: sum, count, min, max, avg Except count, all aggregations apply to a single attribute 50

51 AggregaRon: Count Product (pname, price, category, manufacturer) Company (cname, country) COUNT applies to duplicates, unless otherwise stated: SELECT count(category) FROM Product WHERE price > $20 If category has no nulls, then count(category)=count(*) We probably want: SELECT count(distinct category) FROM Product WHERE price > $20 51

52 Grouping and AggregaRon Product (pname, price, category, manufacturer) Company (cname, country) For each manufacturer, find total number of its products under $200. SELECT manufacturer, count(*) AS total FROM Product WHERE price < $200 GROUP BY manufacturer Let s see what this means 52

53 Grouping and AggregaRon 1. Compute the FROM and WHERE clauses. 2. Group by the attributes in the GROUPBY 3. Compute the SELECT clause, including aggregates. 53

54 SELECT manufacturer, count(*) AS total FROM Product WHERE price < $200 GROUP BY manufacturer 1&2. FROM- WHERE- GROUPBY PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi 54

55 3. SELECT SELECT manufacturer, count(*) AS total FROM Product WHERE price < $200 GROUP BY manufacturer PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi count(*) Manufacturer 2 GizmoWorks 1 Canon 55

56 HAVING Clause Product (pname, price, category, manufacturer) Company (cname, country) Same query, except that we return only those manufacturers that make only products with price > $20 SELECT manufacturer, count(*) AS total FROM Product WHERE price < $200 GROUP BY manufacturer HAVING min(price) > $20 HAVING clause contains conditions on aggregates. 56

57 General form of Grouping and AggregaRon SELECT S FROM R 1,,R n WHERE C1 GROUP BY a 1,,a k HAVING C2 Why? S = may contain asributes a 1,,a k and/or any aggregates but NO OTHER ATTRIBUTES C1 = is any condiron on the asributes in R 1,,R n C2 = is any condiron on aggregate expressions 57

58 General form of Grouping and AggregaRon SELECT FROM WHERE S R 1,,R n C1 GROUP BY a 1,,a k HAVING C2 EvaluaRon steps: 1. Evaluate FROM- WHERE, apply condiron C1 2. Group by the asributes a 1,,a k 3. Apply condiron C2 to each group (may have aggregates) 4. Compute aggregates in S and return the result 58

59 NULLS in SQL Whenever we don t have a value, we can put a NULL Can mean many things: Value does not exists Value exists but is unknown Value not applicable Etc. The schema specifies for each asribute if can be null (nullable asribute) or not How does SQL cope with tables that have NULLs? 59

60 Null Values If x= NULL then 4*(3- x)/7 is srll NULL If x= NULL then x= Joe is UNKNOWN In SQL there are three boolean values: FALSE = 0 UNKNOWN = 0.5 TRUE = 1 60

61 Null Values C1 AND C2 = min(c1, C2) C1 OR C2 = max(c1, C2) NOT C1 = 1 C1 SELECT * FROM Person WHERE (age < 25) AND (height > 6 OR weight > 190) E.g. age=20 heigth=null weight=200 Rule in SQL: include only tuples that yield TRUE 61

62 Null Values Unexpected behavior: SELECT * FROM Person WHERE age < 25 OR age >= 25 Some Persons are not included! 62

63 Null Values Can test for NULL explicitly: x IS NULL x IS NOT NULL SELECT * FROM Person WHERE age < 25 OR age >= 25 OR age IS NULL Now it includes all Persons 63

64 Outerjoins Product (pname, price, category, manufacturer) Company (cname, country) Normally, joins are inner joins : Same as: SELECT x.country, y.pname FROM Company x, Product y WHERE x.cname = y.manufacturer SELECT x.country, y.pname FROM Company x JOIN Product y ON x.cname = y.manufacturer But countries that don t manufacture will not be listed! 64

65 Outerjoins Product (pname, price, category, manufacturer) Company (cname, country) If we want to see the companies that don t produce anything, then we use an outer join: SELECT x.country, y.pname FROM Company x LEFT OUTER JOIN Product y ON x.cname = y.manufacturer 65

66 Company Cname GizmoWorks Canon Hitachi MuseumPass Country USA Japan Japan Vatican Product PName Price Category Manufacturer Gizmo $19.99 Gadgets GizmoWorks Powergizmo $29.99 Gadgets GizmoWorks SingleTouch $ Photography Canon MultiTouch $ Household Hitachi Cname USA USA Japan Japan Vatican PName GizmoWorks GizmoWorks Canon Hitachi NULL 66

67 ApplicaRon Product (pname, price, category, manufacturer) Company (cname, country) Compute the total number of products made by each country SELECT x.country, count(*) FROM Company x, Product y WHERE x.cname = y.manufacturer GROUP BY x.country What s wrong? 67

68 ApplicaRon Product (pname, price, category, manufacturer) Company (cname, country) Compute the total number of products made by each country Note: we don t use count(*) WHY? SELECT x.country, count(y.pname) FROM Company x LEFT OUTER JOIN Product y ON x.cname = y.manufacturer GROUP BY x.country Now we also get the products who sold in 0 quantity 68

69 Outer Joins Leh outer join: Include the leh tuple even if there s no match Right outer join: Include the right tuple even if there s no match Full outer join: Include the both leh and right tuples even if there s no match 69

70 Subqueries A subquery is another SQL query nested inside a larger query Such inner- outer queries are called nested queries A subquery may occur in: 1. A SELECT clause 2. A FROM clause 3. A WHERE clause Rule of thumb: avoid wrirng nested queries when possible; somermes it s impossible 70

71 1. Subqueries in SELECT Product (pname, price, category, manufacturer) Company (cname, country) For each product return the country that manufactures it SELECT X.pname, (SELECT Y.country FROM Company Y WHERE Y.cname=X.manufacturer) FROM Product X What happens if a subquery returns more than one country? 71

72 1. Subqueries in SELECT Product (pname, price, category, manufacturer) Company (cname, country) Whenever possible, don t use a nested queries: SELECT X.pname, (SELECT Y.country FROM Company Y WHERE Y.cname=X.manufacturer) FROM Product X = We have unnested the query SELECT pname, country FROM Product, Company WHERE cname=manufacturer 72

73 1. Subqueries in SELECT Product (pname, price, category, manufacturer) Company (cname, country) Compute the number of products made by each country SELECT DISTINCT x.country, (SELECT count(*) FROM Company y, Product WHERE y.cname=manufacturer and y.country = x.country) FROM Company x Better: we can unnest by using a GROUP BY SELECT x.country, count(*) FROM Company x, Product z WHERE x.cname = z.manufacturer GROUP BY x.country 73

74 GROUP BY v.s. Nested Quereis SELECT manufacturer, count(*) AS total FROM Product WHERE price < '$200 GROUP BY manufacturer SELECT DISTINCT x.manufacturer, (SELECT count(*) FROM Product y WHERE x.manufacturer = y.manufacturer AND price < '$200 ) AS total FROM Product x WHERE price < '$200 Why twice? 74

75 2. Subqueries in FROM Product (pname, price, category, manufacturer) Company (cname, country) Find all products whose prices is > 20 and < 30 SELECT * FROM (SELECT * FROM Product AS Y WHERE Y.price > $20 ) AS x WHERE x.price < $30 Unnest this query! 75

76 3. Subqueries in WHERE Existential quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find all countries that make some products with price < 100 Using EXISTS: Correlated subqery: uses x from outer query SELECT DISTINCT x.country FROM Company x WHERE EXISTS (SELECT * FROM Product y WHERE y.manufacturer = x.cname and y.price < $100 ) 76

77 3. Subqueries in WHERE Existential quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find all countries that make some products with price < 100 Predicate Calculus (a.k.a. First Order Logic) { y x.company(x,y) ( z. p. c.product(z,p,c,x) p<100)} 77

78 3. Subqueries in WHERE Existential quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find all countries that make some products with price < 100 Using IN SELECT DISTINCT country FROM Company WHERE cname IN (SELECT Product.manufacturer FROM Product WHERE Product.price < $100 ) De- correlated subqery 78

79 3. Subqueries in WHERE Existential quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find all countries that make some products with price < 100 Using ANY: SELECT DISTINCT Company.country FROM Company WHERE $100 > ANY (SELECT price FROM Product WHERE manufacturer = cname) 79

80 3. Subqueries in WHERE Existential quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find all countries that make some products with price < 100 Now let s unnest it: SELECT DISTINCT x.country FROM Company x, Product y WHERE x.cname = y.manufacturer and y.price < $100 Existential quantifiers are easy! J 80

81 3. Subqueries in WHERE Universal quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find the countries of all companies that make only products with price < 100 Universal quantifiers are hard! L 81

82 3. Subqueries in WHERE Universal quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find the countries of all companies that make only products with price < 100 Predicate Calculus (a.k.a. First Order Logic) { y x.company(x,y) ( z. p. c.product(z,p,c,x)è p<100) } 82

83 3. Subqueries in WHERE De Morgan s Laws: (A B) = A B (A B) = A B x. P(x) = x. P(x) x. P(x) = x. P(x) (A è B) = A B { y x. Company(x,y) ( z. p. c.product(z,p,c,x)è p<100) } = { y x.company(x,y) ( z p. p.product(z,p,c,x) p 100) } = { y x. Company(x,y)) } { y x. Company(x,y) ( z p. c.product(z,p,c,x) p 100) 83 }

84 3. Subqueries in WHERE 1. Find the other companies: i.e. s.t. some product 100 SELECT DISTINCT country FROM Company WHERE cname IN (SELECT manufacturer FROM Product WHERE price >= $100 ) 2. Find all companies s.t. all their products have price < 100 SELECT DISTINCT country FROM Company WHERE cname NOT IN (SELECT manufacturer FROM Product WHERE price >= $100 ) 84

85 3. Subqueries in WHERE Universal quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find the countries of all companies that make only products with price < 100 Using EXISTS: SELECT DISTINCT x.country FROM Company x WHERE NOT EXISTS (SELECT * FROM Product y WHERE y.manufacturer = x.cname and y.price >= $100 ) 85

86 3. Subqueries in WHERE Universal quantifiers Product (pname, price, category, manufacturer) Company (cname, country) Find the countries of all companies that make only products with price < 100 Using ALL: SELECT DISTINCT Company.country FROM Company WHERE $100 > ALL (SELECT price FROM Product WHERE manufacturer = cname) 86

87 QuesRon for Database Fans and their Friends Find the countries of all companies that make only products with price < 100 Can we unnest this query? 87

88 Monotone Queries A query Q is monotone if: Whenever we add tuples to one or more of the tables the answer to the query cannot contain fewer tuples Fact: all unnested queries are monotone Proof: using the nested for loops semanrcs Fact: A query a universal quanrfier is not monotone Consequence: we cannot unnest a query with a universal quanrfier 88

89 Queries that must be nested Rule of Thumb: Non- monotone queries cannot be unnested. In parrcular, queries with a universal quanrfier cannot be unnested 89

90 More SQL Read the following commands in the book CREATE TABLE INSERT DELETE UPDATE They are easy; but we need/use them all the Rme in class, and in the homework assignments 90

91 Advanced SQLizing 1. UnnesRng Aggregates 2. Finding witnesses 91

92 UnnesRng Aggregates Product (pname, price, category, manufacturer) Company (cname, country) For each category, find the maximum price SELECT DISTINCT X.category, (SELECT max(y.price) FROM Product Y WHERE X.category = Y.category) FROM Product X SELECT category, max(price) FROM Product GROUP BY category Equivalent queries Note: no need for DISTINCT (DISTINCT is the same as GROUP BY)

93 UnnesRng Aggregates Product (pname, price, category, manufacturer) Company (cname, country) Find the number of products made in each country SELECT DISTINCT X.country, (SELECT count(*) FROM Company Y, Product Z WHERE Y.cname=Z.manufacturer AND Y.country = X.country) FROM Company X SELECT X.country, count(*) FROM Company X, Product Y WHERE X.cname=Y.manufacturer GROUP BY X.country They are NOT equivalent! (WHY?)

94 Author(login,name) Wrote(login,url) More UnnesRng Find authors who wrote 10 documents: ASempt 1: with nested queries SELECT DISTINCT Author.name FROM Author WHERE This is SQL by a novice count(select Wrote.url FROM Wrote WHERE Author.login=Wrote.login) > 10 94

95 More UnnesRng Find all authors who wrote at least 10 documents: ASempt 2: SQL style (with GROUP BY) SELECT DISTINCT Author.name FROM Author, Wrote WHERE Author.login=Wrote.login GROUP BY Author.name HAVING count(wrote.url) > 10 This is SQL by an expert 95

96 Finding Witnesses Product (pname, price, category, manufacturer) Company (cname, country) For each country, find its most expensive products 96

97 Finding Witnesses Product (pname, price, category, manufacturer) Company (cname, country) For each country, find its most expensive products Finding the maximum price is easy SELECT x.country, max(y.price) FROM Company x, Product y WHERE x.cname = y.manufacturer GROUP BY x.country But we need the witnesses, i.e. the products with max price

98 Finding Witnesses To find the witnesses, compute the maximum price in a subquery SELECT u.country, v.pname, v.price FROM Company u, Product v, (SELECT x.country, max(y.price) as mprice FROM Company x, Product y WHERE x.cname = y.manufacturer GROUP BY x.country) AS p WHERE u.country = p.country and v.price = p.mprice 98

99 Finding Witnesses Product (pname, price, category, manufacturer) Company (cname, country) There is a more concise solution here: SELECT x.country, y.pname, y.price FROM Company x, Product y WHERE x.cname = y.manufacturer and y.price >= ALL (SELECT z.price FROM Product z WHERE x.cname = z.manufacturer) 99