The Structured Query Language

Zachary G. IvesUniversity of Pennsylvania

CIS 550 – Database & Information Systems

September 27, 2005

Some slide content courtesy of Susan Davidson & Raghu Ramakrishnan

2

Administrivia

Homework 2 handed out today Due in 1 week, 10/4

Please sign up for Oracle accounts ASAP http://www.seas.upenn.edu/ora/ You can do the homework without this, but:

Will be useful in testing your HW2’s! You’ll need to get familiar with Oracle anyway

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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

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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))

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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

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Exercise

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

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Revised Example Data Instance

sid name

1 Jill

2 Qun

3 Nitin

4 Marty

fid name

1 Ives

2 Saul

8 Martin

sid exp-grade

cid

1 A 550-0105

1 A 700-1005

3 A 700-1005

3 C 501-0105

4 C 501-0105

cid subj sem

550-0105 DB F05

700-1005 AI S05

501-0105 Arch F05

555-1006 Sys S06

fid cid

1 550-0105

2 700-1005

8 501-0105

STUDENT Takes COURSE

PROFESSOR Teaches

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Nested Queries in SQL

Simplest: IN/NOT IN

Example: Students who have taken subjects that have (at any point) been taught by Martin

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Correlated Subqueries

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

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Universal and Existential Quantification

Generally used with subqueries: {op} ANY, {op} ALL Find the students with the best expected

grades

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Table Expressions

Can substitute a subquery for any relation in the FROM clause:

SELECT S.sidFROM (SELECT sid FROM STUDENT WHERE sid = 5) SWHERE S.sid = 4

Notice that we can actually simplify this query!

What is this equivalent to?

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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

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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

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Data Instance, Again

sid name

1 Jill

2 Qun

3 Nitin

4 Marty

fid name

1 Ives

2 Saul

8 Martin

sid exp-grade

cid

1 A 550-0105

1 A 700-1005

3 A 700-1005

3 C 501-0105

4 C 501-0105

cid subj sem

550-0105 DB F05

700-1005 AI S05

501-0105 Arch F05

555-1006 Sys S06

fid cid

1 550-0105

2 700-1005

8 501-0105

STUDENT Takes COURSE

PROFESSOR Teaches

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What If You Want to Only ShowSome 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 CWHERE S.sid = T.sid AND T.cid = C.cidGROUP BY subjHAVING COUNT(S.sid) > 5

Exercise: For each subject taught by at least two professors, list the minimum expected grade

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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 sizeFROM STUDENT S, Takes T, COURSE CWHERE S.sid = T.sid AND T.cid =

C.cidGROUP BY cid, subj)

GROUP BY subj

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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?

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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 123-4567

NULL

Li 234-8972

234-8766

Maria

789-2312

789-2121SELECT * FROM CONTACT WHERE Fax < “789-1111”

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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 = UF AND U = FU AND U = U

T OR U = TF OR U = UU OR U = U

NOT U = U

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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.cidWHERE …

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Data Instance, Again (!)

sid name

1 Jill

2 Qun

3 Nitin

4 Marty

fid name

1 Ives

2 Saul

8 Martin

sid exp-grade

cid

1 A 550-0105

1 A 700-1005

3 A 700-1005

3 C 501-0105

4 C 501-0105

cid subj sem

550-0105 DB F05

700-1005 AI S05

501-0105 Arch F05

555-1006 Sys S06

fid cid

1 550-0105

2 700-1005

8 501-0105

STUDENT Takes COURSE

PROFESSOR Teaches

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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

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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)

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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

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Deleting Tuples

Deletion is a fairly simple operation:

DELETEFROM STUDENT SWHERE S.sid < 25

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Updating Tuples

What kinds of updates might you want to do?

UPDATE STUDENT SSET S.sid = 1 + S.sid, S.name = ‘Janet’WHERE S.name = ‘Jane’

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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[])

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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, ageINTO :sid, :nameFROM STUDENTWHERE sid < 20

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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;

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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);…

}

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Database-Backed Web Sites We all know traditional static HTML web

sites:Web-Browser

HTTP-Request

GET ...

Web-Server

File-System

Load File

HTML-File

HTML-File

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Common Gateway Interface (CGI)

Can have the web server invoke code (with parameters) to generate HTML

Web ServerHTTP-Request

HTML-File

Web Server

File-SystemLoad File

FileHTML?

HTML

Execute Program

Program?Output

I/O, Network, DB

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CGI: Discussion

Advantages: 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

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Java-Server-Process

DB Access in Java

Sybase

Java Applet

TCP/UDP

IP

Oracle ...

JDBC-Driver

JDBC-Driver

JDBC-Driver

JDBC Driver manager

BrowserJVM

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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)

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*SP Server Pages and Servlets(IIS, Tomcat, …)

File-SystemWeb Server

HTTP Request

HTML File

Web Server

Load File

FileHTML?

HTML

I/O, Network, DB

Script/Servlet?

Output

Server Extension

May have a built-in VM (JVM, CLR)

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DB-Driven Web Server

One Step Beyond: DB-Driven Web Sites (Strudel, Cocoon, …)

LocalDatabase

HTTP Request

HTML File

Web Server

Cache

Data

HTML

Other datasources

Script?

DynamicHTML

Generation

Styles

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Wrapping Up

We’ve seen how to query in SQL (DML) 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