CRIMEWard

location WARD

CrimeRef.

N 1Deprivation

X

Grid ref.

Ward code Name

HO_code

Name

Populationdate

Y

DISTRICTIn district

N

1

Crimedescriptor

Date charged

Pub. Id.

Pub. name

Offender id.

DoB

CRIME TYPE

Home office code

1

N

OFFENDER

PUBLIC HOUSE

Perpetrated by

X

Grid ref.

Y

X

Home Grid ref.

Y

Ethnicity

Pub in ward

Lives in

N

M

1

N

N

1

Gender

FINAL ERM FOR CRIME DATABASE (CHEN NOTATION)

DDI 3: From ERM to relational database schema

• Reminder of RDBMS concepts first:– Primary key (PK) – uniquely identifies each row in table– Foreign key (FK) – a column that provides a link to the PK of

another table, FK-PK matches mediate “joins”– Attributes (columns) must be single-valued (i.e. no lists of values

in cells – e.g. keywords) and atomic (i.e. not complex/compound)• Rules for translating ERM to RDBMS:

– Entities tables– Attributes columns

• 1 column for each component of complex attribute• Multi-valued attributes need table of their own

– 1-to-1 and 1-to-many: add FKs to existing tables– Many to many relationships: need table of their own with two FKs

linking the two entity types together

TABLES FOR THE CRIME DATABASE (“database schema”) DERIVED FROM THE ERM

NB: No table needed for DISTRICT as no specific data is required

Join attributes are indicated, i.e. FK/PK links

Normalization – slightly simplified• Functional dependency (FD):– An attribute (X, the determinant ) functionally determines

another (Y, the dependent attribute ) if the X value necessarily “fixes” the value of the Y attribute

– E.g. HO_code -> crime_descriptor, Offender_ID -> DoB– NB: vice versa generally not the case

• Relations (tables) should always be in something called Boyce-Codd Normal Form (BCNF)– Informally, this means that there should be no FDs

amongst the non-key attributes– i.e. all non-key attributes should be functionally

dependent only on the primary key

Normalization cont’d• For example, if we had the following table for CRIMES:

– Crime ref., HO_code, crime_descriptor, date etc.– This would not be in BCNF, because HO_code-> crime_descriptor

• This is problematic in two ways: – Information is redundantly repeated (i.e. the crime_descriptor)

wasting space– Updating is complicated. If, for instance, one of the crime_descriptors

were reworded, every mention of it would have to be changed, rather than a single instance as in the normalized version

• Doing an ERM should mean your database schema is in good shape. It is especially helpful in spotting many-to-many relationships, which always require a table of their own

• But (here is the moral): – Always check your tables and if an FD is found amongst the non-key

attributes, extract the FD and put it in a table of its own– Think of this as a health check!

Integrity rules• Entity integrity:– Every entity must have a primary key

• Referential integrity:– There must be a matching value for every value of a

foreign key in the relation “pointed at” by the FK – NB. The FK can have a NULL value

• Both of these integrity rules will be automatically enforced by the RDBMS, e.g.– Inserting a record with a NULL PK will not be allowed, nor

a duplicate value– Inserting a record with a value for a FK, for which no

matching PK value can be found, will not be allowed

CREATING TABLES IN MS ACCESS

CREATE TABLE CRIME_TYPE (HO_CODE varchar(4) PRIMARY KEY, CRIME_DESCRIPTOR varchar(100)) CREATE TABLE CRIME(CRIME_REF varchar(8) PRIMARY KEY, HO_code varchar(4),FOREIGN KEY (HO_code) REFERENCES CRIME_TYPE(HO_code), CRIME_DATE date,X int,Y int,WARD_CODE varchar(8))

The following SQL commands set up two of the tables we need for the CRIME database (NB. SQL is the general purpose language for manipulating data in a RDBMS, including creating tables and adding data) :

NB – the CRIME_TYPE table has to be created first. Why?

NB2 – WARD_CODE not yet designated as a FK, this can be done later with an ALTER TABLE command

NB3 – there is actually a “wizard” for creating tables in Access, which you will use in the lab class

PS. Multi-valued attributes

• Key word is multi-valued – an arbitrarily ordered list of one to several words. Need two tables:– BOOK: ISBN, Author, Title– KEYWORD: ISBN, keyword

• The latter has a separate entry for each keyword for a given book

BOOK

Key word

TitleISBN

Author