Database Design

Sections 11 & 12drawing conventions, generic model, integrity, keys, mapping conceptual model to logical/physical model

Marge Hohly 2

Conventions Review Crows feet

Crows fly East and South Divide complex ERD’s into functional areas Place Highest volume entities in upper left

corner Improve readability

avoid criss-crossing lines increase white spaces so relationships don’t

overlap be consistent with font type, size, and styles

Marge Hohly 3

Generic Modeling Can reduce number of entities in diagram Can provide more flexibility in unstable

situations (where business requirements change often)

Use a more distant perspective Review 11.3.3 What would happen to the generic model if

we had to add 10 new ARTICLE types, each with their own attributes?

Marge Hohly 4

Generic Modeling Have more attributes in fewer entities Many mandatory requirements/attributes

become optional Structural rules become procedural rules Example: PANTS waist size was mandatory,

with ARTICLE waist size becomes optional What other businesses would be good

candidates for generic modeling?

Marge Hohly 5

Relational Database Concepts

Conceptual model transforms into a relational database

A relational database is a database that is perceived by the user as a collection of relations or two-dimensional tables.

Table, each employee (instances), and each column (attribute)

Marge Hohly 6

SQL to retrieve information

Structured query language (SQL) used to access information

English-like phrases Example:

SELECT lname, dept_noFROM employeesWHERE emp_no = 210;

Marge Hohly 7

Using HTML_DB SQL editor

HTML_DB SQL Editor

Marge Hohly 8

Table Definitions

Table – Dfn.

Marge Hohly 9

Employee Table Structure

Marge Hohly 10

Selecting all records

SELECT * FROM employeesWHERE department_no = 10;

Marge Hohly 11

Keys Primary Key (PK)

not null no part of PK can be null (entity integrity) unique can be composite

Foreign Key (FK) depends on business rule comes from relationship primary key from another table If FK is part of a PK, then the FK can’t be NULL

Marge Hohly 12

Key questions

11.4.8 what makes emp_no and payroll_id good candidates for the primary key?

11.4.9 why is having alternate or unique keys useful?

Marge Hohly 13

Referential Integrity

Use Foreign Key to map relationships A foreign key (FK) is a column or

combination of columns in one table that refers to a primary key in the same table or another table.

11.4.10 (next slide)

Marge Hohly 14

11.4.10

Marge Hohly 15

Composite key

Made up of two or more values Together unique ENROLL Table/Entity

student_no & ticket_no ACCOUNTS

bank_no & acct_no

Marge Hohly 16

JOBS Table

Marge Hohly 17

Data-Integrity Summary Entity integrity- no part of PK can be NULL Referential integrity – FK must match an

existing PK value (or else be NULL) Column integrity – column must contain

only values consistent with defined data format

User-defined integrity – data stored in database must comply with the rules of the business

Marge Hohly 18

Transformation

Conceptual model, focus on the business and its rules.

Data modeling pays attention to the business requirements, regardless of implementation.

Conceptual model Logical model

Marge Hohly 19

Review 12.2.3

Marge Hohly 20

Conceptual becomes Physical model

Conceptional becomes Physical model

Marge Hohly 21

Terminology Mapping

- An entity leads to a table. - An attribute becomes a column. - A primary unique identifier

produces a primary key. - A secondary unique identifier

produces a unique Key. - A relationship is transformed

into a foreign key and foreign-key columns.

- Constraints are the rules that the database must follow to be consistent. Some of the business rules are translated into check constraints; other more complex ones require additional programming in the database or the application.

Marge Hohly 22

12.2.8 For entity names of more than one word, take

the: - First character of the first word - First character of the second word - Last character of the last word Example: JOB ASSIGNMENT gets a short

name of JAT

For entity names of one word but more than one syllable, take the:

- First characer of the first syllable - First character of the second syllable - Last character of the last syllable Example: EMPLOYEE gets a short name of

EPE

For entity names of one syllable but more than one character:

- First character - Second character - Last character Example: FLIGHT gets a short name of FLT

Marge Hohly 23

Naming restrictions with Oracle Table and column names:

must start with a letter can contain up to 30 alphanumeric

characters cannot contain space or special characters

such as “!,” but “$,” “#,” and “-“ are permitted

Table names must be unique. Column names must be unique within a

table. Avoid “reserved” words in tables and

columns.

Marge Hohly 24

Cascade barred relationships

UID from parent entity becomes part of the UID of the child entity

Marge Hohly 25

Relationship mapping

Relationships are mapped to foreign keys

Foreign keys enable users to access related information from other tables.

Mapping relationships to relational database structures is part of creating the “first-cut” database design.

Marge Hohly 26

Relationship mapping 1:M mapping Foreign key goes in

table at crow’s foot from parent

FK1 Dept_id mandatory is required

FK2 might be better mgn_id and is optional

Does the president of the company have a manager?

Marge Hohly 27

Relationship mapping

FK is mandatory from this diagram

FK is optional from this diagram

Marge Hohly 28

12.3.4 Optional or

Mandatory determined by crow’s foot end of relationship

Marge Hohly 29

NonTransferable Relationship

Transferablility is a procedural model Must be implemented by a program Need to document this

constraint/business rule

Marge Hohly 30

Barred Relationship 12.3.6 Barred relationship is mapped to a

foreign-key column on the many side, just like any other M:1 relationship.

Bar means it becomes part of the composite primary key of the child

ACCOUNT table has both acct_id and bank_id as the composite primary key

Marge Hohly 31

Cascading barred relationships Pick up one more

component to the composite key with each level

Company – company_id

Division company_id & div_id

Department company_id, div_id & dept_no

Team team_id, company_id, div_id & dept_no

TEAM

DEPARTMENT

DIVISION

COMPANY

within

within

within

made up of

made up of

made up of

Marge Hohly 32

M:M relationship mapping M:M resolved with

intersection entity Intersection entity

has a composite key with the PK from each parent as FK in child

Marge Hohly 33

1:1 relationship mapping

Create a foreign key and a unique key If relationship mandatory on one side,

Foreign key created on the mandatory side as a unique key

If optional on both sides, you can choose which table gets the foreign key.

Marge Hohly 34

Review

FK 1:M

PK, FK in same key, rename one

M:M first resolve with an intersection entity

*

o

Marge Hohly 35

Review cont.

Will be part of PK a composite key

FK on mandatory side

FK on either side

Marge Hohly 36

Arc mapping Foreign key from the parent (single)

side are placed in the child (many) side

The Foreign key is ALWAYS Optional in the child

Only of the Arc can be valid and all others must be NULL

Mandatory relationship is enforced with a check constraint

Marge Hohly 37

Arc constraint You need a constraint to make sure

only one is NOT NULL at a time Example: FK1, FK2, FK3, .... ALTER EVENT constraint (FK1 is not

null and FK2 is null and FK3 is null ....) OR (FK1 is null and FK2 is not null and FK3 is null ....) OR (FK1 is null and FK2 is null and FK3 is not null ....)

Marge Hohly 38

ARC mapping

If mandatory then one MUST be NOT NULL

If optional then all may be NOT NULL You will always need a check

constraint defined

Marge Hohly 39

Subtype Review

Marge Hohly 40

Subtype mapping

Mapping supertypes and subtypes makes sure that the right information gets stored with each type.

Marge Hohly 41

Subtype modeling Mapping as a single table Rules

Tables: Only one table is created, independent of the number of subtypes.

Columns: The single table gets a column for all the attributes of the supertype, with the original optionality.

Table gets a column for each attribute of the subtype, but column are.

Mandatory column to distinguish between each different subtypes of entity.

Marge Hohly 42

Subtype modeling – Single table cont.

Rules Identifiers: Unique identifiers transform into

primary and unique keys. Relationships: Relationships at the supertype

level transform as usual. Relationships at subtype level are implemented as optional foreign-key columns.

Integrity constraints: A check constraint is needed to ensure that for each particular subtype, all columns that come from mandatory attributes are not null.

Marge Hohly 43

Subtype model – Single table Note mandatory

attributes salary/hourly rate became optional

Need check constraint to enforce mandatory requirement CHECK (epe_type =

‘FTE’ and salary is not null and hourly_rate is null and agy_id is null) OR (epe_type ‘PTE’ and salary is null and hourly_rate is not null and agy_id is not null)

Marge Hohly 44

When Supertype/Single table

The single-table implementation is common and flexible implementation.

Appropriate where: Most attributes are at supertype level Most relationships are at supertype level Business rules are globally the same for

the subtypes

Marge Hohly 45

Two-Table implementation Create a table for each subtype Rules

Tables: One table per first-level subtype.

Columns: Each table gets a column for all attributes of the supertype with the original optionality.

Each table also gets a column for each attribute belonging to the subtype, also with the original optionality.

Identifiers: The primary UID at the supertype level creates a primary key for each table. Secondary UIDs of the supertype become unique keys in each table.

Relationships: All tables get a foreign key for a relationship at the supertype level, with the original optionality. For relationships at the subtype levels, the foreign key is implemented in the table it is mapped to. Original optionality is retained.

Marge Hohly 46

2-table cont. A separate table

would be created for SHIRTS and SHOES.

Marge Hohly 47

Subtype Considerations Subtype implementation may be

appropriate when: Subtypes have very little in common. There are

few attributes at the supertype level and several at the subtype level.

Most of the relationships are at the subtype level.

Business rules and functionality are quite different between subtypes.

How tables are used is different -- for example, one table is being queried while the other is being updated.