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Database Conceptual and Logical Design
Zachary G. IvesUniversity of Pennsylvania
CIS 550 – Database & Information Systems
October 4, 2005
Some slide content courtesy of Susan Davidson & Raghu Ramakrishnan
2
Administrivia
Homework 2 due now
Homework 3 handed out (due on the 13th)
3
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
4
Deleting and Modifying Tuples
Deletion is a fairly simple operation:
DELETEFROM STUDENT SWHERE S.sid < 25
So is insertion:
UPDATE STUDENT SSET S.sid = 1 + S.sid, S.name = ‘Janet’WHERE S.name = ‘Jane’
5
I’m Building an App: 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 Dynamic: ODBC, JDBC, ADO, OLE DB, …
Typically, predefined mappings between host language types and SQL types (e.g., VARCHAR String or char[])
6
The Impedance Mismatch and Cursors
SQL is set-oriented – it returns relations There’s no relation type in most
languages! Solution: result sets and cursors that are
opened, read, as if from a file
7
JDBC: Dynamic SQL for Java
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);…
}
8
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
9
Interaction Is Achieved via HTML Forms
<html><form action=“http://my.com/some-
handler-url” method=“POST”><input type=“text” name=“value1” /><input type=“submit” value=“Send” /><input type=“rest” value=“Cancel” />
</form>
10
Java-Server-Process
DB Access with JavaApplets and Server Processes
Sybase
Java Applet
TCP/UDP
IP
Oracle ...
JDBC-Driver
JDBC-Driver
JDBC-Driver
JDBC Driver manager
BrowserJVM
11
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)
A common alternative is to run code on the server-side CGI, ASP/PHP/JSP, ASP.Net, servlets
12
Server Pages (*P) and Servlets(IIS, Tomcat, …)
File-SystemWeb Server
HTTP Request
HTML File
Web Server
Load File
FileHTML?
HTML
I/O, Network, DB
Script?Output
Server Extension
13
ASP/JSP/PHP “Escapes”
<html><head><title>Sample</title></head><body><h1>Sample</h1><%
myClass.Process(request.getParameter(“test”)); %>
<%= request.getParameter(“value”); %></body></html>
14
Servlets
class MyClass extends HttpServlet {public void doGet(HttpRequest req, HttpResponse res) … {
…
res.println(“<html><head><title>Test</title></head></html>”);}
}
15
ASP/JSP/PHP Versus Servlets
The goal: combine direct HTML (or XML) output with program code that’s executed at the server
The code is responsible for generating more HTML, e.g., to output the results of a database table as HTML table elements
How might I do this? HTML with embedded code (*P) Code that prints out HTML (Servlets)
16
Now: How Do We Get the Database in the First Place?
Database design theory!
Neat outcome: we can actually prove that we have optimal design, in a manner of speaking…
But first we need to understand how to visualize in pretty pictures…
17
Databases Anonymous:A 6-Step Program
1. Requirements Analysis: what data, apps, critical operations
2. Conceptual DB Design: high-level description of data and constraints – typically using ER model
3. Logical DB Design: conversion into a schema4. Schema Refinement: normalization
(eliminating redundancy)5. Physical DB Design: consider workloads,
indexes and clustering of data6. Application/Security Design
18
Entity-Relationship Diagram(based on our running example)
STUDENTS COURSESTakes
namesid serno subj
PROFESSORS
Teaches
cid
fid name
entity set relationship set
exp-grade
attributes (recall these have domains)
Underlined attributes are keys
semester
19
Conceptual Design Process
What are the entities being represented?
What are the relationships?
What info (attributes) do we store about each?
What keys & integrity constraints do we have?
name
STUDENTS
Takes
sid
exp-grade
20
Translating Entity Sets toLogical Schemas & SQL DDL
CREATE TABLE STUDENTS (sid INTEGER, name VARCHAR(15) PRIMARY KEY (sid) )
CREATE TABLE COURSES (serno INTEGER, subj VARCHAR(30), cid CHAR(15), PRIMARY KEY (serno) )
Fairly straightforward to generate a schema…
21
Translating Relationship Sets
Generate schema with attributes consisting of: Key(s) of each associated entity (foreign keys) Descriptive attributes
CREATE TABLE Takes (sid INTEGER, serno INTEGER, exp-grade CHAR(1), PRIMARY KEY (?), FOREIGN KEY (serno) REFERENCES COURSES, FOREIGN KEY (sid) REFERENCES STUDENTS)
22
… OK, But What about Connectivityin the E-R Diagram?
Attributes can only be connected to entities or relationships
Entities can only be connected via relationships
As for the edges, let’s consider kinds of relationships and integrity constraints…
COURSESPROFESSORS Teaches
(warning: the book has a slightly different notation here!)
23
Logical Schema Design
Roughly speaking, each entity set or relationship set becomes a table (not always be the case; see Thursday)
Attributes associated with each entity set or relationship set become attributes of the relation; the key is also copied (ditto with foreign keys in a relationship set)
24
Binary Relationships & Participation
Binary relationships can be classified as 1:1, 1:Many, or Many:Many, as in:
1:1 1:n m:n
25
1:Many (1:n) Relationships
Placing an arrow in the many one direction, i.e. towards the entity that’s ref’d via a foreign key
Suppose profs teach multiple courses, but may not have taught yet:
Suppose profs must teach to be on the roster:
COURSESPROFESSORS Teaches
COURSESPROFESSORS Teaches
Partial participation (0 or more…)
Total participation (1 or more…)
26
Many-to-Many Relationships
Many-to-many relationships have no arrows on edges The “relationship set” relation has a key that
includes the foreign keys, plus any other attributes specified as key
STUDENTS COURSESTakes
27
Examples
Suppose courses must be taught to be on the roster
Suppose students must have enrolled in at least one course
28
Representing 1:n Relationships in Tables
CREATE TABLE Teaches( fid INTEGER, serno CHAR(15), semester CHAR(4), PRIMARY KEY (serno), FOREIGN KEY (fid) REFERENCES PROFESSORS, FOREIGN KEY (serno) REFERENCES Teaches)
CREATE TABLE Teaches_Course( serno INTEGER, subj VARCHAR(30), cid CHAR(15), fid CHAR(15), when CHAR(4), PRIMARY KEY (serno), FOREIGN KEY (fid) REFERENCES PROFESSORS)
• Key of relationship set:
• Or embed relationship in “many” entity set:
29
1:1 Relationships
If you borrow money or have credit, you might get:
What are the table options?
CreditReport Borrower
delinquent?
ssn
namedebt
Describesrid
30
Roles: Labeled Edges
Sometimes a relationship connects the same entity, and the entity has more than one role:
This often indicates the need for recursive queries
name
qty
Partsid
Assembly Subpart
Includes
31
DDL for Role ExampleCREATE TABLE Parts (Id INTEGER, Name CHAR(15), … PRIMARY KEY (ID) )
CREATE TABLE Includes (Assembly INTEGER, Subpart INTEGER, Qty INTEGER, PRIMARY KEY (Assemb, Sub), FOREIGN KEY (Assemb) REFERENCES Parts, FOREIGN KEY (Sub) REFERENCES Parts)
32
Married
Roles vs. Separate Entities
Husband Wifeid id
Husband Wife
name name
What is the differencebetween these two representations?
Married
Personid
name
33
ISA Relationships: Subclassing(Structurally)
Inheritance states that one entity is a “special kind” of another entity: “subclass” should be member of “base class”
name
ISA
Peopleid
Employees salary
34
But How Does this Translateinto the Relational Model?
Compare these options: Two tables, disjoint tuples Two tables, disjoint attributes One table with NULLs Object-relational databases
35
Weak Entities
A weak entity can only be identified uniquely using the primary key of another (owner) entity. Owner and weak entity sets in a one-to-many
relationship set, 1 owner : many weak entities Weak entity set must have total
participation
People Feeds Pets
ssn name weeklyCost name species
36
Translating Weak Entity Sets
Weak entity set and identifying relationship set are translated into a single table; when the owner entity is deleted, all owned weak entities must also be deleted
CREATE TABLE Feed_Pets ( name VARCHAR(20), species INTEGER, weeklyCost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (pname, ssn), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE)
37
N-ary Relationships
Relationship sets can relate an arbitrary number of entity sets:
Student Project
Advisor
IndepStudy
38
Summary of ER Diagrams
One of the primary ways of designing logical schemas
CASE tools exist built around ER (e.g. ERWin, PowerBuilder, etc.) Translate the design automatically into DDL,
XML, UML, etc. Use a slightly different notation that is better
suited to graphical displays Some tools support constraints beyond what ER
diagrams can capture Can you get different ER diagrams from the
same data?
39
Schema Refinement & Design Theory
ER Diagrams give us a start in logical schema design
Sometimes need to refine our designs further There’s a system and theory for this Focus is on redundancy of data
Let’s briefly touch on one key concept in preparation for Thursday’s lecture on normalization…
40
Not All Designs are Equally Good
Why is this a poor schema design?
And why is this one better?
Stuff(sid, name, cid, subj, grade)
Student(sid, name)Course(cid, subj)Takes(sid, cid, exp-grade)
41
Focus on the Bad Design
Certain items (e.g., name) get repeated Some information requires that a student be
enrolled (e.g., courses) due to the key
sid
name
cid
subj
exp-grade
1 Sam 570
AI B
23 Nitin 550
DB A
45 Jill 505
OS A
1 Sam 505
OS C
42
Functional DependenciesDescribe “Key-Like” Relationships
A key is a set of attributes where:If keys match, then the tuples match
A functional dependency (FD) is a generalization:If an attribute set determines another, written A ! B
then if two tuples agree on A, they must agree on B:
sid ! Address
What other FDs are there in this data?
FDs are independent of our schema design choice
43
Formal Definition of FD’s
Def. Given a relation scheme R (a set of attributes) and subsets X,Y of R:An instance r of R satisfies FD X Y if,
for any two tuples t1, t2 2 r, t1[X ] = t2[X ] implies t1[Y] = t2[Y]
For an FD to hold for scheme R, it must hold for every possible instance of r
(Can a DBMS verify this? Can we determine this by looking at an instance?)
44
General Thoughts on Good Schemas
We want all attributes in every tuple to be determined by the tuple’s key attributesWhat does this say about redundancy?
But: What about tuples that don’t have keys (other
than the entire value)? What about the fact that every attribute
determines itself?
Stay tuned for Thursday!