1. M.P. Johnson, DBMS, Stern/NYU, Spring 20051 C20.0046: Database Management Systems Lecture #15 M.P. Johnson Stern School of Business, NYU Spring, 2005

2. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 2 Summary: SQL queries Only SELECT, FROM required Can’t have HAVING without GROUP BY Can have GROUP BY without HAVING Any clauses used must appear in this order: SELECTL FROMRs WHEREs GROUP BYL2 HAVINGs2 ORDER BYL3 SELECTL FROMRs WHEREs GROUP BYL2 HAVINGs2 ORDER BYL3

3. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 3 Deletions General form: E.g.: DELETE FROM Table WHERE condition DELETE FROM Table WHERE condition INSERT INTO Knights VALUES('R. Giuliani', 'n', 'KBE'); INSERT INTO Knights VALUES('Bernard Kerik', 'n', 'CBE'); DELETE FROM Knights WHERE name = 'Bernard Kerik'; INSERT INTO Knights VALUES('R. Giuliani', 'n', 'KBE'); INSERT INTO Knights VALUES('Bernard Kerik', 'n', 'CBE'); DELETE FROM Knights WHERE name = 'Bernard Kerik';

4. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 4 Updates General form: Example: UPDATE Product SET field1 = value1, field2 = value2 WHERE condition UPDATE Product SET field1 = value1, field2 = value2 WHERE condition UPDATE Product SET price = price/2 WHERE Product.name IN (SELECT product FROM Purchase WHERE Date = DATE'Oct, 25, 1999') UPDATE Product SET price = price/2 WHERE Product.name IN (SELECT product FROM Purchase WHERE Date = DATE'Oct, 25, 1999')

5. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 5 Default Values Specify defaults when creating table: The default default: NULL CREATE TABLE People ( name VARCHAR(30), ssn CHAR(9), age SHORTINT DEFAULT 100, city VARCHAR(30) DEFAULT 'New York', gender BIT(1), dob DATE DEFAULT DATE '1900-01-01' ) CREATE TABLE People ( name VARCHAR(30), ssn CHAR(9), age SHORTINT DEFAULT 100, city VARCHAR(30) DEFAULT 'New York', gender BIT(1), dob DATE DEFAULT DATE '1900-01-01' )

6. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 6 Deleting and modifying schemata Delete data, indices, schema: Delete data and indices: Either way, exercise extreme caution! Add or delete attributes: Q: What’s put in the new fields? DROP TABLE Person TRUNCATE TABLE Person ALTER TABLE Person ADD phone CHAR(12) ALTER TABLE Person ADD phone CHAR(12) ALTER TABLE Person DROP age ALTER TABLE Person DROP age

7. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 7 Creating Indices Syntax: Here: Searches are speed up logarithmically If all New Yorkers, #comparisons:  8000000  log 2 (8000000) ~= 23  (i.e., 2 23 ~= 8000000) In general:  Indices speed up queries a lot  They slow down modifications somewhat They also speed up some modifications… CREATE INDEX index-name ON R(fields) CREATE INDEX nameIndex ON Person(name)

8. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 8 Another BST example

9. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 9 Constraints in SQL Constraints in SQL: Keys foreign keys Attribute-level constraints Tuple-level constraints Global constraints: assertions The more complex the constraint, the harder it is to check and to enforce simplest Most complex

10. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 10 Primary Keys & Unique Keys There is at most one PRIMARY KEY; there can be many UNIQUE  Primary key v. candidate keys Key fields get indices automatically (why?) CREATE TABLE Product ( productID CHAR(10), name CHAR(30), category VARCHAR(20), price INT, PRIMARY KEY (productID), UNIQUE (name, category) ) CREATE TABLE Product ( productID CHAR(10), name CHAR(30), category VARCHAR(20), price INT, PRIMARY KEY (productID), UNIQUE (name, category) )

11. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 11 prodName is a foreign key to Product(name) name should be a key in Product Purchase ~ Product is many-one NB: referenced field specified with (), not dot Referenced and referring fields should/must be indexed (why?) Foreign Key Constraints CREATE TABLE Purchase ( prodName CHAR(30) REFERENCES Product(name), date DATETIME ) CREATE TABLE Purchase ( prodName CHAR(30) REFERENCES Product(name), date DATETIME ) Referential integrity in SQL

12. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 12 Constraints on Attributes and Tuples Constraints on attributes:  NOT NULL-- obvious meaning... Can combine NOT NULL with foreign key constraints  CHECK condition-- any condition on row itself Some DBMS support subqueries here, but many don’t Constraints on tuples  CHECK condition

13. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 13 CREATE TABLE Purchase ( prodName CHAR(30) CHECK (prodName IN SELECT Product.name FROM Product), date DATETIME NOT NULL ) CREATE TABLE Purchase ( prodName CHAR(30) CHECK (prodName IN SELECT Product.name FROM Product), date DATETIME NOT NULL ) How is this different from a Foreign Key? Tuple-level constraint

14. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 14 General Assertions Defined in latest SQL standard, but doesn’t really exist Implemented/approximated in MySQL and Oracle as stored procedures  PL/SQL in Oracle CREATE ASSERTION myAssert CHECK (NOT EXISTS( SELECT Product.name FROM Product, Purchase WHERE Product.name = Purchase.prodName GROUP BY Product.name HAVING count(*) > 200) ) CREATE ASSERTION myAssert CHECK (NOT EXISTS( SELECT Product.name FROM Product, Purchase WHERE Product.name = Purchase.prodName GROUP BY Product.name HAVING count(*) > 200) )

15. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 15 Final Comments on Constraints Can give them names, and alter later or drop  Still possible if unnamed, but more work to find Can define constraints  On fields – NOT NULL, CHECK  On tuples  As triggers that look at multiple tables  On whole databases (in theory)

16. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 16 Final Comments on Constraints Each more powerful than last, but more expensive to enforce  Like indices On the other hand:  The more mistakes you make impossible, the better  The more mistakes you make impossible on the DBMS side, the fewer you have to check on the client side Think of data validation

17. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 17 Live examples Q: produce a list of employees and their bosses  What if no boss? Or no subordinate? Joins on emp, emp man:  Comma-based  Cross  Natural  Inner Must use INNER JOIN in MySQL  If want non-managers, do outer join… No FULL OUTER JOIN in MySQL yet

18. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 18 Live examples Q: produce list of bosses and underling- counts, for bosses with >1 underling Just add HAVING clause…

19. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 19 Finally: R.A./SQL has limitations Can easily find Alice’s direct subordinates: But: find all of King’s underlings Cannot compute “transitive closure” Cannot express in R.A./SQL! SQL is not “Turing-Complete” NameJobBoss KingPresidentNULL JonesManagerKing BlakeManagerKing FordAnalystJones ScottAnalystJones

20. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 20 Midterm Know all steps of project so far:  Draw E/R from dataset description  Convert E/R to relations  Normalize badly designed relations Indices:  Why important  How BSTs work Constraints  Why important  Basic concepts SQL! http://pages.stern.nyu.edu/~mjohnson/dbms/dbmsmtinfo.html

21. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 21 Midterm course evaluations Please do midterm evals on Blackboard! Worth 50 points on the midterm Longish, but very helpful for me Vote on interest levels for remaining possible topics “We’re in touch, so you be in touch.” Thanks!

22. M.P. Johnson, DBMS, Stern/NYU, Spring 2005 22 Live examples Examples from sqlzoo.netsqlzoo.net  L (  C (R 1 x … R n ) SELECT L FROM R 1, …, R n WHERE C SELECT L FROM R 1, …, R n WHERE C