1. How to build ER diagrams
CS 622/215 Databases
How to build ER diagrams
Will use an example of a bank database from Elmasri as an example
Banks have branches
Customers have accounts.
May be joint Accounts
Possibly multiple accounts.
Eg: savings account, checking account
Customers may also take out loans
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2. How to build ER diagrams
CS 622/215 Databases
How to build ER diagrams
Identify entities
Including weak entities
Identify attributes
Identify relationships
Some attributes might become relationships
Determine structure of relationships
Partial/total
cardinality
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3. How to build ER diagrams
CS 622/215 Databases
How to build ER diagrams
Identify attributes of relationships
Might move  attributes of entities
Figure out keys
Details of attributes
Simple/composite
Single valued/multi-valued
Role names for recursive relationships
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4. Elmasri FIGURE 7.21: ER DIAGRAM FOR A BANK DATABASE
© The Benjamin/Cummings Publishing Company, Inc. 1994, Elmasri/Navathe, Fundamentals of Database Systems, Second Edition

5. ER to Relational: Regular entities
CS 622/215 Databases
ER to Relational: Regular entities
ER to Relational:
Given an ER diagram : high level view of database
Build a Relational Schema
For regular entity E, create a relation R. Attributes ?
Will include the attributes of E.
Eg: DEPT
Primary key in R ?
Pick one of the keys of E as the primary key for R.
If the chosen key of E is composite ?
Then R will also have the same composite key.
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6. Regular entities What are other regular entities ?
CS 622/215 Databases
Regular entities
CREATE TABLE DEPT
( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, PRIMARY KEY (DNUMBER), UNIQUE (DNAME));
What are other regular entities ?
Create tables EMPLOYEE. PROJECT.
Primary keys ?
EMPLOYEE : SSN
PROJECT : PNUMBER
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7. Composite attributes Composite attribute:
CS 622/215 Databases
Composite attributes
Composite attribute:
Eg: Name: LName, MI, FName
Have as different columns in table
CREATE TABLE EMP ( FNAME VARCHAR(15) MINIT CHAR, LNAME VARCHAR(15) SSN CHAR(9));
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8. Weak Entity Weak entity WK with owner entity OW. How?
CS 622/215 Databases
Weak Entity
Weak entity WK with owner entity OW. How?
Create a relation R and include all attributes of WK as attributes of R. PK of R?
The PK of R is the combination of the PK of OW and the partial key of the weak entity WK.
In addition, include as a foreign key of R the primary key attribute of OW. If OW deleted?
When OW is deleted, WK must also be deleted.
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9. Weak Entity CREATE TABLE DEPENDENT ( ESSN CHAR(9) NOT NULL,
CS 622/215 Databases
Weak Entity
CREATE TABLE DEPENDENT (
ESSN CHAR(9) NOT NULL,
DNAME VARCHAR(15),
SEX CHAR,
PRIMARY KEY (DNAME, ESSN),
FOREIGN KEY (ESSN) REFERENCES EMPLOYEE (SSN),
ON DELETE CASCADE)
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10. Many-many relationship
CS 622/215 Databases
Many-many relationship
many-many relationship R between entities E and F.
Eg: WORKSON
Can R be represented in the table for E?
No: one E can be connected to many F.
For same reason, can’t be in table for F. How to do?
Create a new table to represent R.
What will be the PK of this table?
Composite PK: the PK from each table
Eg: SSN, PNUMBER
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11. Many-many relationship
CS 622/215 Databases
Many-many relationship
Attributes of relationship type R?
Will become columns of table R
Eg: HOURS in WORKSON table
CREATE TABLE WorksOn (
ESSN CHAR(9),
PNO INTEGER,
HOURS DECIMAL,
PRIMARY KEY (ESSN, PNO),
FOREIGN KEY (ESSN)
REFERENCES EMPLOYEE (SSN),
FOREIGN KEY (PNO)
REFERENCES PROJECT(PNUMBER)
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12. One-many relationship
CS 622/215 Databases
One-many relationship
Eg: WORKSFOR
1:N relationship R, between E (1) and F (N).
Eg: 1 for DEPT, N for EMP
Options?
Create a new table or
Put R info in the table for F: advantage is we won’t have to look up 2 tables
PK will stay the same.
Foreign Key to E
Include attributes of R in this table
Store DNUMBER or DNAME in EMP table?
Store PK from other table
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13. One-many, recursive relationship
CS 622/215 Databases
One-many, recursive relationship
CREATE TABLE EMP ( ESSN CHAR(9), BDATE DATE, DNO INTEGER, SUPERSSN CHAR(9), PRIMARY KEY (ESSN), FOREIGN KEY (DNO) REFERENCES DEPT (DNUMBER), FOREIGN KEY (SUPERSSN) REFERENCES EMPLOYEE (SSN),);
Recursive relationship: supervision: is having supervisor in supervisee table better than having supervisee in supervisor table?
Yes: one-many
Foreign Key to PK in same table
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14. 1-1 relationship 1:1 relationship R, between E and F
CS 622/215 Databases
1-1 relationship
1:1 relationship R, between E and F
Eg: MANAGES ,1 for EMP, 1 for DEPT
Create a new table or
Put R info in E table or in F table
Could put in EMP or DEPT. Which is better?
DEPT: every DEPT has a Manager, but not every EMP is a manager. Difference?
Won’t need NULLS if in DEPT table
Generally put in side of total participation
Attribute of R (eg: MgrStartDate) also here
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15. CS 622/215 Databases
1-1 relationship
CREATE TABLE DEPT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, MGRSSN CHAR(9), MGRSTARTDATE CHAR(9), PRIMARY KEY (DNUMBER), UNIQUE (DNAME), FOREIGN KEY (MGRSSN) REFERENCES EMP (SSN) );
Is it possible that even for a 1-1 relationship we might want a separate table?
Yes, if only a few instances of relationship and not total on either side. Avoids NULLs.
Disadvantage of separate table: lots of JOINS.
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16. Multi-valued attributes
CS 622/215 Databases
Multi-valued attributes
Eg: DEPT_LOCATIONS. How to do?
Can it go in DEPT table?
No: for one department can have multiple values
Needs a separate table. Primary key?
PK: DNUMBER, DLOCATION. Foreign key?
FK: DNUMBER
Multi-valued attributes need separate table
Composite PK
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17. FIGURE 3.7 Result of mapping the COMPANY ER schema into a relational schema.

18. Participation Constraints
CS 622/215 Databases
Participation Constraints
Can we capture total participation in relational schema?
Eg: every DEPT has to have a manager
Eg: every EMP has to work in a DEPT
Can this be done in relational schema ?
No : can’t capture this in relational schema . In SQL ?
By specifying NOT NULL
CREATE TABLE EMP ( ESSN CHAR(9), BDATE DATE, DNO INTEGER NOT NULL);
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19. Ternary relationships: Elmasri Eg
CS 622/215 Databases
Ternary relationships: Elmasri Eg
Elmasri FIGURE 7.17 (a) SUPPLY relationship
How to do?
Create a new table. Primary keys?
PK: composite: the PK of all the 3 entities.
FK: to all 3 entities
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20. Elmasri FIGURE 9.4: Mapping the n-ary relationship type SUPPLY

21. Correspondence between ER and relational models Table 9.1

22. Build relational schema for the bank database from Figure 7.21
© The Benjamin/Cummings Publishing Company, Inc. 1994, Elmasri/Navathe, Fundamentals of Database Systems, Second Edition

23. Informal Design Guidelines for Databases
CS 622/215 Databases
Informal Design Guidelines for Databases
Relational database design
Create "good" relation schema
Criteria for "good" relation schema? 
First we discuss informal guidelines.
Then discuss formal concepts of functional dependencies and normal forms
1NF (First Normal Form)
2NF (Second Normal Form)
BCNF (Boyce-Codd Normal Form)
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24. Informal Guidelines for Relations
Basic design principle: Design a schema such that
What each table stands for should be easy to explain
Semantics of attributes should be easy to interpret/explain.
Each tuple in a relation should represent one entity or relationship instance.
Attributes of different entities (EMPLOYEE, DEPARTMENT, PROJECT) should not be mixed in the same relation
Only foreign keys used to refer to other entities

25. How to design a database
CS 622/215 Databases
How to design a database
Build the ER diagram.
Figure out what data is being stored
The connections between different data
Translate ER diagram to relational schema
keeping informal guidelines in mind
Run through normalization checks
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26. Eg of bad relational design
CS 622/215 Databases
Eg of bad relational design
Eg: Students pay same amount for same course
Tuition across different courses varies
Suppose store information in a single table:
Which students take which courses.
What is the tuition for a course
Howmuch ( studentid, course# , tuition)
Stid c# tuition
Problems with this design ?
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27. Eg of bad relational design
CS 622/215 Databases
Eg of bad relational design
Redundancy: tuition repeated for students taking the same course. Problem with redundancy ?
Wasted storage.
Because of the duplication, can get inconsistency:
If we make changes to tuition
622’s tuition 1150 in one place
1100 in another.
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28. Modification Anomalies
CS 622/215 Databases
Modification Anomalies
Modification anomalies – unexpected and undesirable behavior when making changes
Update Anomaly: if 622’s tuition changed to 1140 ?
Have to make this change in many places.
Same information is in multiple rows
Deletion Anomaly: if 345 leaves ?
We lose tuition info for 640.
Lose info we would like to keep
Insertion Anomaly: Can insert 632’s fee is 1175 ?
No : because no students registered for 632
Can’t store info we would like to store
How to fix ?
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29. Relational Design/Modification Anomalies
CS 622/215 Databases
Relational Design/Modification Anomalies
Problem: too much info in one table
Has to be split.
How to split ?
Whotakes( stid,course#), Cost( course#,tuition)
Does this solve all the problems:
Redundancy
Update anomaly
Insertion anomaly
Deletion anomaly
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30. Non-loss decomposition
CS 622/215 Databases
Non-loss decomposition
Problem: how do we know when a table can be split into two tables without loss of information?
Non-loss decomposition: no info loss if table split up
what does this mean in specific terms
If we break table up, should be able to recover original table with join
without spurious tuples
new rows which were not there in original table.
Lossy decomposition : we lose info when we split table. Eg: spurious tuples when we do join
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31. Non-loss/lossy decomposition eg
CS 622/215 Databases
Non-loss/lossy decomposition eg
(S#,Status,City) broken up into
(S#,Status) and (S#, City).
Is this a lossy or non-loss decomposition
Can recover original table with join. How do we know this ?
S# is primary key
Non-loss but no advantage of doing this. Disadvantage ?
Need a join if we wanted to answer:
Which cities have S with status 20
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32. Non-loss/lossy decomposition eg
CS 622/215 Databases
Non-loss/lossy decomposition eg
(S#,Status,City) broken up into : (S#,Status) and (Status, City).
S Status City
S1 10 SF
S2 20 NY
S3 20 LA
S Status
S1 10
S2 20
S3 20
Status City SF NY LA
Will the join (over status) of the two new tables give back the original table?
No: join will also have spurious tuples
S LA
S3 20 NY
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33. Normalization Want to design/modify tables to control redundancy
CS 622/215 Databases
Normalization
Want to design/modify tables to control redundancy
To avoid modification anomalies and wasted storage.
This process is called Normalization
Look at a given table structure and see if it can be improved.
Is there potential for anomalies because too much info in one table ?
How to break up ? What do we want ?
Non-loss decomposition
Any downside to normalization?
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34. CS 622/215 Databases
Normalization
By splitting up tables, may lead to loss of efficiency. Why ?
Because more joins needed
Normalization is a guide
Don’t always have to follow
But have to understand
Works through stages, called Normal Forms
each is stronger than the previous
i.e. the higher the normal form, the stronger the guarantee of less redundancy.
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