1. M.P. Johnson, DBMS, Stern/NYU, Spring 20051 C20.0046: Database Management Systems Lecture #10 Matthew P. Johnson Stern School of Business, NYU Spring, 2005

2. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 2 Next topic: SQL 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/MySQL  “No one ever got fired for buying 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)

3. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 3 SQL Query Semantics SELECT a1, a2, …, ak FROM R1 AS x1, R2 AS x2, …, Rn AS xn WHERE Conditions Parallel assignment – all tuples 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

4. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 4 SQL Query Semantics SELECT a1, a2, …, ak FROM R1 AS x1, R2 AS x2, …, Rn AS xn WHERE Conditions 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

5. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 5 Details: 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?

6. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 6 Details: Disambiguation in SQL Every selected field must be unambiguous For R(A,B),  Select A from R, R   Select R1.A from R R1, R R2 Consider: Why? * is shorthand for all fields, each must be unambiguous  Select * from R R1, R R2 SQL> Select * from R, R; Select * from R, R * ERROR at line 1: ORA-00918: column ambiguously defined SQL> Select * from R, R; Select * from R, R * ERROR at line 1: ORA-00918: column ambiguously defined

7. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 7 Details: Tuple Variables SELECT DISTINCT x.store AS competitor FROM Purchase AS x, Purchase AS y WHERE x.product = y.product AND y.store = 'BestBuy' SELECT DISTINCT x.store AS competitor 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)

8. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 8 Details: Disambiguation in Oracle SQL Can rename fields by  Select name as n …  Select name n … But not by  Select name=n… Can rename relations only by  … from tab t1, tab t2 Lesson: if you get errors, remove all =s, ASs

9. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 9 R.A. & SQL Reps(ssn, name, etc.) Clients(ssn, name, rssn) Q: Who are George’s clients, in R.A.?   Clients.name (  Reps.name='George' and Reps.ssn=rssn (Reps x Clients)) In SQL?

10. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 10 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

11. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 11 Ordering the Results SELECTCategory FROMProduct ORDER BYPName SELECTCategory FROMProduct ORDER BYPName PNamePriceCategoryManufacturer Gizmo$19.99GadgetsGizmoWorks Powergizmo$29.99GadgetsGizmoWorks SingleTouch$149.99PhotographyCanon MultiTouch$203.99HouseholdHitachi ?

12. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 12 Details: Case-sensitivity By default, all matches and comparisons are case- sensitive If want case-insensitive, some options: Convert all to upper or lower case - slow  SQL> select * from emp where upper(ename) = upper(‘Blake'); Create a function index  Maybe later… Modify the nls_sort setting:  SQL> alter session set nls_sort=binary_ci;  SQL> alter session set nls_comp=ansi;  The other values: binary, binary_ai

13. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 13 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%’

14. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 14 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%’ ESCAPE ‘x’

15. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 15 Details: More on escape chars SQL: no official default escape char In SQL*Plus: default escape char = '\'  Can set with  SQL> set escape x Other tools, DBMSs: your mileage may vary SQL string literals put in ‘ ‘:  'mystring' Single-quote literals escaped with single- quotes:  'George''s string'

16. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 16 Details: More on escape chars Q: Can an escape char be an escape string? A: No. SQL> select * from newtable where a like '%\%' escape '\'; A B ---------- h%i there SQL> select * from newtable where a like '%\%' escape '\\'; select * from newtable where a like '%\%' escape '\\' * ERROR at line 1: ORA-01425: escape character must be character string of length 1 SQL> select * from newtable where a like '%\%' escape '\'; A B ---------- h%i there SQL> select * from newtable where a like '%\%' escape '\\'; select * from newtable where a like '%\%' escape '\\' * ERROR at line 1: ORA-01425: escape character must be character string of length 1

17. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 17 Details: More on single-quotes Dates with DATE:  DATE '1948-05-14' Timestamps with TIMESTAMP:  TIMESTAMP '1948-05-14 12:00:00' Details may vary by DBMS

18. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 18 Details: More on quotes Q: What about double quotes? A: Can’t be used in place of single quotes But can be used when Oracle would otherwise misparse your command, e.g.: 1. Names with spaces:  create table bad table name (a int, b int); 2. Reserved words as names:  create table badfieldname(from int, b int);

19. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 19 Complex RA/SQL Expressions Reps(ssn, name, etc.) Clients(ssn, name, rssn) Q: Who are George’s clients?  Clients.name (  Reps.name='George' and Reps.ssn=rssn ( Reps x Clients)) Or:  Clients.name ( Reps.ssn=rssn (  Reps.name='George' (Reps) x Clients))

20. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 20 Complex RA Expressions People(ssn, name, street, city, state, state) Q: Who lives on George’s street? A: First, find George:   name='George' (People) Get George’s street/city/state:   street,city,state (  name='George' (People)) Join with People:  People x  street,city,state (  name='George' (People))

21. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 21 Complex RA Expressions How to specify street = street? Rename   p2(s2,c2) (People) x  street,city (  name='George' (People)) Now can select:  street=s2 AND city=c2 (  p2(s2,c2) (People) x  street,city (  name='George' (People))) Then project names… Only way? No. Join!  People   street,city (  name='George' (People)) Q: Would the following work?   street,city (  name='George' (People  People))

22. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 22 R.A.  SQL People(ssn, name, street, city, state)  assume for clarity that cities are unique Q: Who lives on George’s street? In R.A.:  street=s2 AND city=c2 (  p2(s2,c2) (People) x  street,city (  name='George' (People))) In SQL? The other way in R.A.: People  street,city (  name='George' (People)) In SQL? Later on…

23. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 23 Complex RA Expressions Scenario: 1. Purchase(pid, seller-ssn, buyer-ssn, etc.) 2. Person(ssn, name, etc.) 3. Product(pid, name, etc.) Q: Who (give names) bought gizmos from Dick? Where to start? Purchase uses pid, ssn, so must get them…

24. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 24 Complex RA Expressions Person Purchase Person Product  name='Dick'  name='Gizmo'  pid  ssn seller-ssn=ssnpid=pidbuyer-ssn=Person.ssn  name