1. The Structured Query Language
Zachary G. Ives
University of Pennsylvania
CIS 550 – Database & Information Systems
September 28, 2004
Some slide content courtesy of Susan Davidson & Raghu Ramakrishnan

2. Administrivia Homework 2 handed out today
Please sign up for Oracle accounts ASAP
Will be useful in testing your HW2’s
Google talk at 3:00 this afternoon – Craig Neville-Manning, “Finding needles in a 20 TB haystack, 200 million times per day”
Wu and Chen auditorium, Levine 101

3. Recall Basic SQL
SELECT [DISTINCT] {T1.attrib, …, T2.attrib} FROM {relation} T1, {relation} T2, … WHERE {predicates}
SELECT *
All STUDENTs AS
As a “range variable” (tuple variable): optional
As an attribute rename operator
select-list
from-list
qualification

4. Expressions in SQL
Can do computation over scalars (int, real or string) in the select-list or the qualification
Show all student IDs decremented by 1
Strings:
Fixed (CHAR(x)) or variable length (VARCHAR(x))
Use single quotes: ’A string’
Special comparison operator: LIKE
Not equal: <>
Typecasting:
CAST(S.sid AS VARCHAR(255))

5. Set Operations
Set operations default to set semantics, not bag semantics:
(SELECT … FROM … WHERE …) {op} (SELECT … FROM … WHERE …)
Where op is one of:
UNION
INTERSECT, MINUS/EXCEPT (many DBs don’t support these last ones!)
Bag semantics: ALL

6. Exercise Find all students who have taken DB but not AI
Hint: use EXCEPT

7. Example Data Instance STUDENT Takes COURSE PROFESSOR Teaches sid name1
Jill 2
Qun 3
Nitin 4
Marty
sid
exp-grade
cid 1 A 3 C 4
cid
subj
sem DB
F03 AI
S03
Arch
PROFESSOR
Teaches
fid
name 1
Ives 2
Saul 8
Roth
fid
cid 1 2 8

8. Nested Queries in SQL Simplest: IN/NOT IN
Example: Students who have taken subjects that have (at any point) been taught by Roth

9. Correlated Subqueries
Most common: EXISTS/NOT EXISTS
Find all students who have taken DB but not AI

10. Universal and Existential Quantification
Generally used with subqueries:
{op} ANY, {op} ALL
Find the students with the best expected grades

11. Table Expressions
Can substitute a subquery for any relation in the FROM clause:
SELECT S.sid FROM (SELECT sid FROM STUDENT WHERE sid = 5) S WHERE S.sid = 4
Notice that we can actually simplify this query!
What is this equivalent to?

12. Aggregation
GROUP BY
SELECT {group-attribs}, {aggregate-operator}(attrib) FROM {relation} T1, {relation} T2, … WHERE {predicates} GROUP BY {group-list}
Aggregate operators
AVG, COUNT, SUM, MAX, MIN
DISTINCT keyword for AVG, COUNT, SUM

13. Some Examples Number of students in each course offering
Number of different grades expected for each course offering
Number of (distinct) students taking AI courses

14. Example Data Instance STUDENT Takes COURSE PROFESSOR Teaches sid name1
Jill 2
Qun 3
Nitin 4
Marty
sid
exp-grade
cid 1 A 3 C 4
cid
subj
sem DB
F03 AI
S03
Arch
PROFESSOR
Teaches
fid
name 1
Ives 2
Saul 8
Roth
fid
cid 1 2 8

15. What If You Want to Only Show Some Groups?
The HAVING clause lets you do a selection based on an aggregate (there must be 1 value per group):
SELECT C.subj, COUNT(S.sid) FROM STUDENT S, Takes T, COURSE C WHERE S.sid = T.sid AND T.cid = C.cid GROUP BY subj HAVING COUNT(S.sid) > 5
Exercise: For each subject taught by at least two professors, list the minimum expected grade

16. Aggregation and Table Expressions (aka Derived Relations)
Sometimes need to compute results over the results of a previous aggregation: SELECT subj, AVG(size) FROM ( SELECT C.cid AS id, C.subj AS subj, COUNT(S.sid) AS size FROM STUDENT S, Takes T, COURSE C WHERE S.sid = T.sid AND T.cid = C.cid GROUP BY cid, subj) GROUP BY subj

17. Thought Exercise… Tables are great, but…
Not everyone is uniform – I may have a cell phone but not a fax
We may simply be missing certain information
We may be unsure about values
How do we handle these things?

18. One Answer: Null Values
We designate a special “null” value to represent “unknown” or “N/A”
But a question: what does: do?
Name
Home
Fax
Sam
NULL Li
Maria
SELECT * FROM CONTACT WHERE Fax < “ ”

19. Three-State Logic
Need ways to evaluate boolean expressions and have the result be “unknown” (or T/F)
Need ways of composing these three-state expressions using AND, OR, NOT:
Can also test for null-ness: attr IS NULL, attr IS NOT NULL
Finally: need rules for arithmetic, aggregation
T AND U = U F AND U = F U AND U = U
T OR U = T F OR U = U U OR U = U
NOT U = U

20. Nulls and Joins Sometimes need special variations of joins:
I want to see all courses and their students
… But what if there’s a course with no students?
Outer join:
Most common is left outer join:
SELECT C.subj, C.cid, T.sid FROM COURSE C LEFT OUTER JOIN Takes T ON C.cid = T.cid WHERE …

21. Revised Example Data Instance
STUDENT
Takes
COURSE
sid
name 1
Jill 2
Qun 3
Nitin 4
Marty
sid
exp-grade
cid 1 A 3 C 4
cid
subj
sem DB
F03 AI
S03
Arch
Food
PROFESSOR
Teaches
fid
name 1
Ives 2
Saul 8
Roth
fid
cid 1 2 8

22. Warning on Outer Join
Oracle doesn’t support standard SQL syntax here: SELECT C.subj, C.cid, T.sid FROM COURSE C , Takes T WHERE C.cid =(+) T.cid

23. Beyond Null
Can have much more complex ideas of incomplete or approximate information
Probabilistic models (tuple 80% likely to be an answer)
Naïve tables (can have variables instead of NULLs)
Conditional tables (tuple IF some condition holds)
… And what if you want “0 or more”?
In relational databases, create a new table and foreign key
But can have semistructured data (like XML)

24. Modifying the Database: Inserting Data
Inserting a new literal tuple is easy, if wordy: INSERT INTO PROFESSOR(fid, name) VALUES (4, ‘Simpson’)
But we can also insert the results of a query! INSERT INTO PROFESSOR(fid, name) SELECT sid AS fid, name FROM STUDENT WHERE sid < 20

25. Deleting Tuples
Deletion is a fairly simple operation: DELETE FROM STUDENT S WHERE S.sid < 25

26. Updating Tuples
What kinds of updates might you want to do? UPDATE STUDENT S SET S.sid = 1 + S.sid, S.name = ‘Janet’ WHERE S.name = ‘Jane’

27. Now, How Do I Talk to the DB?
Generally, apps are in a different (“host”) language with embedded SQL statements
Static: SQLJ, embedded SQL in C
Runtime: ODBC, JDBC, ADO, OLE DB, …
Typically, predefined mappings between host language types and SQL types (e.g., VARCHAR  string or char[])

28. Embedded SQL in C EXEC SQL BEGIN DECLARE SECTION
int sid; char name[20];
EXEC SQL END DECLARE SECTION …
EXEC SQL INSERT INTO STUDENT
VALUES (:sid, :name);
EXEC SQL
SELECT name, age
INTO :sid, :name
FROM STUDENT
WHERE sid < 20

29. The Impedance Mismatch and Cursors
SQL is set-oriented – it returns relations
There’s no relation type in most languages!
Solution: cursor that’s opened, read
DECLARE sinfo CURSOR FOR SELECT sid, name FROM STUDENT … OPEN sinfo; while (…) { FETCH sinfo INTO :sid, :name … } CLOSE sinfo;

30. JDBC: Dynamic SQL Roughly speaking, a Java version of ODBC
See Chapter 6 of the text for more info import java.sql.*; Connection conn = DriverManager.getConnection(…); PreparedStatement stmt = conn.prepareStatement(“SELECT * FROM STUDENT”); … ResultSet rs = stmt.executeQuery (); while (rs.next()) { sid = rs.getInteger(1); … }

31. Database-Backed Web Sites
We all know traditional static HTML web sites:
Web-Browser
Web-Server
HTTP-Request
GET ...
HTML-File
File-System
Load File
All pages are static
Need to generate web pages on the fly depending on user input

32. Common Gateway Interface (CGI)
Can have the web server invoke code (with parameters) to generate HTML
Web Server
Web Server
File-System
HTTP-Request
Load File
HTML
HTML?
HTML-File
Execute Program
Program?
Output
File
I/O, Network, DB

33. CGI: Discussion Advantages: Disadvantages:
Standardized: works for every web-server, browser
Flexible: Any language (C++, Perl, Java, …) can be used
Disadvantages:
Statelessness: query-by-query approach
Inefficient: new process forked for every request
Security: CGI programmer is responsible for security
Updates: To update layout, one has to be a programmer

34. DB Access in Java Browser JVM Java Applet Java-Server-Process
TCP/UDP IP
Java-Server-Process
JDBC Driver manager
JDBC-Driver
JDBC-Driver
JDBC-Driver
Sybase
Oracle
...

35. Java Applets: Discussion
Advantages:
Can take advantage of client processing
Platform independent – assuming standard java
Disadvantages:
Requires JVM on client; self-contained
Inefficient: loading can take a long time ...
Resource intensive: Client needs to be state of the art
Restrictive: can only connect to server where applet was loaded from (for security … can be configured)

36. *SP Server Pages and Servlets (IIS, Tomcat, …)
Web Server
Web Server
HTTP Request
File-System
Load File
HTML
HTML?
HTML File
File
Script?
Output
Server Extension
I/O, Network, DB

37. One Step Beyond: DB-Driven Web Sites (Strudel, Cocoon, …)
Web Server
DB-Driven Web Server
HTTP Request
Styles
Cache
HTML File
HTML
Script?
Local Database
Dynamic HTML Generation
Data
Other data sources

38. Wrapping Up We’ve seen how to query in SQL (DML)
Basic foundation is TRC-based
Subqueries and aggregation add extra power
Nulls and outer joins add flexibility of representation
We can update tables
We’ve seen that SQL doesn’t precisely match standard host language semantics
Embedded SQL
Dynamic SQL
Data-driven web sites