1. The Structured Query Language
Zachary G. Ives
University of Pennsylvania
CIS 550 – Database & Information Systems
November 9, 2018
Some slide content courtesy of Susan Davidson & Raghu Ramakrishnan

2. Administrivia Preparation for Homework 2 (handed out next week)
We have Oracle set up on eniac.seas.upenn.edu
There will be a test data set for your HW2 queries
Go to: Click on “create Oracle account” link Enter your login info so you’ll get an Oracle account

3. Basic SQL: A Friendly Face Over the Tuple Relational Calculus
SELECT [DISTINCT] {T1.attrib, …, T2.attrib} FROM {relation} T1, {relation} T2, … WHERE {predicates}
Let’s do some examples, which will leverage your knowledge of the relational calculus…
Faculty ids
Course IDs for courses with students expecting a “C”
Courses taken by Jill
select-list
from-list
qualification

4. Our 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
F09 AI
S09
Arch
PROFESSOR
Teaches
fid
name 11
Ives 12
Taskar 18
Martin
fid
cid 11 12 18

5. Some Nice Shortcuts SELECT * AS Example: All STUDENTs
As a “range variable” (tuple variable): optional
As an attribute rename operator
Example:
Which students (names) have taken more than one course from the same professor?

6. 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))

7. 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

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

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

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

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

12. 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?

13. 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

14. 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

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
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. Data Instance with a Minor Modification
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
F09 AI
S09
Arch
PROFESSOR
Teaches
fid
name 11
Ives 12
Taskar 18
Martin
fid
cid 11 12 18

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 (query fixed): SQLJ, embedded SQL in C
Dynamic (query generated by program at runtime): ODBC, JDBC, ADO, OLE DB, …
Predefined mappings between SQL types and host language types
CHAR, VARCHAR  String
INTEGER  int
DOUBLE  double

28. Static SQL using SQLJ int sid = 5; String name5 = "Jim", name5;
// Database connection setup omitted
#sql {
INSERT INTO STUDENT
VALUES(:sid, :name) };
SELECT name INTO :name6 FROM STUDENT
WHERE sid = 6

29. JDBC: Dynamic SQL import java.sql.*;
Connection conn = DriverManager.getConnection(…);
Statement s = conn.createStatement();
int sid = 5;
String name = "Jim";
s.executeUpdate("INSERT INTO STUDENT VALUES(" + sid + ", '" + name + "')");
// or equivalently
s.executeUpdate(" INSERT INTO STUDENT VALUES(5, 'Jim')");

30. Static vs. Dynamic SQL Syntax Execution
Static is cleaner that Dynamic
Dynamic doesn’t extend language syntax, so you can use any tool you like
Execution
Static must be precompiled
Can be faster at runtime
Extra step is needed to deploy application
Static checks SQL syntax at compilation time, Dynamic at run time
We’ll focus on JDBC, since it’s easy to use

31. 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
ResultSet rs = stmt.executeQuery("SELECT * FROM STUDENT");
while (rs.next()) {
int sid = rs.getInt("sid");
String name = rs.getString("name");
System.out.println(sid + ": " + name); }

32. JDBC: Prepared Statements (1)
But query compilation takes a (relatively) long time!
This example is therefore inefficient.
int[] students = {1, 2, 4, 7, 9}; for (int i = 0; i < students.length; ++i) { ResultSet rs = stmt.executeQuery("SELECT * " +
"FROM STUDENT WHERE sid = " + students[i]); while (rs.next()) { … }

33. JDBC: Prepared Statements (2)
To speed things up, prepare statements and bind arguments to them
This also means you don’t have to worry about escaping strings, formatting dates, etc.
Problems with this lead to a lot of security holes (SQL injection)
Or suppose a user inputs the name “O’Reilly”
PreparedStatement stmt = conn.prepareStatement("SELECT * " +
"FROM STUDENT WHERE sid = ? "); int[] students = {1, 2, 4, 7, 9}; for (int i = 0; i < students.length; ++i) { stmt.setInt(1, students[i]); ResultSet rs = stmt.executeQuery(); while (rs.next()) { … }

34. Language Integrated Querying (LINQ) – MS .Net Languages, e.g., C#
Represent a table as a collection (e.g., a list)
Integrate SQL-style select-from-where and allow for iterators
List products = GetProductList();
var expensiveInStockProducts =
from p in products
where p.UnitsInStock > 0 && p.UnitPrice > 3.00M
select p;
Console.WriteLine("In-stock products costing > 3.00:");
foreach (var product in expensiveInStockProducts) {
Console.WriteLine("{0} in stock and costs > 3.00.", product.ProductName); }

35. 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

36. 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

37. 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

38. 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
...

39. 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)

40. *SP Server Pages and Servlets (IIS, Tomcat, …)
Web Server
Web Server
HTTP Request
File-System
Load File
HTML
HTML?
HTML File
File
Script/ Servlet?
Output
Server Extension
I/O, Network, DB
May have a built-
in VM (JVM, CLR)

41. 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

42. Wrapping Up We’ve seen how to query in SQL
Basic foundation is TRC-based
Subqueries and aggregation add extra power beyond *RC
Nulls and outer joins add flexibility of representation
We can update tables
We’ve also seen that SQL doesn’t precisely match standard host language semantics
Embedded SQL
Dynamic SQL
We’ve seen a hint of data-driven web site architectures