THE HIERARCHICAL DATA MODEL

SECTION 9

Fundamentals

Introduction

• Relational model– Grounded in mathematics

• Network model– Evolved from effort to establish standards

• Hierarchical– Evolved?

Some Hierarchical Systems

• Most widely used

• Other Hierarchical Systems

• IMS

• Developed in early 1960s

• The reason?

• Still remains an actively used DB

Basic Concepts

• Special case of the network model

• Parent

• Child

• Tree

• Hierarchical data model

• Segment types

• Parent-child relationship type

A conceptual model

RETIREMENTPLAN

EMPLOYEE

HAS-EMPLOYEE

PROVIDED-FOR

1

M M

1

DEPARTMENT

A network data model

DEPARTMENTRETIREMENT

PLAN

EMPLOYEE

DEPARTMENT/

EMPLOYEE SET

RETIREMNT PLAN/

EMPLOYEE SET

A hierarchical data model

DEPARTMENTRETIREMENT

PLAN

EMPLOYEE

DEPARTMENT/

EMPLOYEE PCR

RETIREMNT PLAN/

EMPLOYEE PCR

EMPLOYEE

Tree Structure Conventions

2. Every segment a child

3. A parent can have many children

4. A child can only have one parent

1. Root segment

6. Ancestors and dependent segments

7. Root of a sub-tree

5. A leaf segment

Hierarchical arrangement of segment types

DEPTNAME MANAGER

EMPNAME YRS-SVC

PROJNAME DIVISIONSKILLCODE SKILLNAME

DEPARTMENT

EMPLOYEE

SKILL ASSIGNEDTO

Segment type

Hierarchical occurrence tree

Accounting BlackMarketing Smith

DEPTNAME MANAGER

Engineering Cook

Bond 7

EMPNAME YRS-SVC

DEPARTMENT

EMPLOYEE

Mills 12Hill 5 Carter 3 Blake 6 Case 7 West 6

Programmer2

SKILLCODE

SKILL

SKILLNAME

Analyst5 Proj.A 3 Cost Acct.3 Engineer5 Proj.B 6

PROJNAME DIV

ASSIGNEDTO

Preorder Traversal Method

2. At any point, after recording a segment, record the leftmost child segment of the segment just recorded. If the segment has no child segment, move back up the tree one level and record the leftmost unrecorded child segment of the segment at that level. Continue until all segments are accounted for.

1. Begin at the root of the tree and record the root segment in a file

A

B C D

FE G H I J K

D

E E E

S S SA A A A

Segment type indicator

Department Segment Type

Employee Segment Type

Skill and AssignedTo Segment Type

Level 1

Level 2

Level 3

Illustration of Preorder Traversal

• The following file will be generatedIteration Segment Segment-Type Indicator

1 A D

2 B E

3 E S

4 F A

5 G A

6 C E

7 H S

8 I A

9 D E

10 J S

11 K A

The Models Relationship to Conceptual Modeling• Transforming a conceptual model to a

hierarchical data structure

• One - Many Relationships

Transforming One-Many Relationships

Very simple

DEPARTMENT EMPLOYEEHAS-

EMPLOYEE1 N

DEPTNAME LOCATION MANAGER EMPNAME ADDRESS YRS-SVC

DEPTNAME LOCATION MANAGERDEPARTMENT

EMPNAME ADDRESS YRS-SVCEMPLOYEE

• Redundancy problems

DEPARTMENTRETIREMENT

PLAN

HAS-EMPLOYEE

1

N

DEPTNAME LOCATION MANAGER TYPE ENROLL-DATE AMOUNT

PROVIDED-FOR

EMPLOYEE

YRS-SVCADDRESSEMPNAME

1

N

• What is the problem?

DEPTNAME LOCATION MANAGER

DEPARTMENT

EMPNAME ADDRESS YRS-SVC

EMPLOYEE

TYPEENROLL-

DATEAMOUNT

RETIREMENT-PLAN

EMPNAME ADDRESS YRS-SVC

EMPLOYEE

1. For each entity set E in a conceptual model, create a segment type S in the hierarchical model. All attributes of E are represented as fields of S.

2. For one-many relationships between two entity sets, create corresponding tree structure diagrams, making each entity a segment and making the one-many relationship a parent-child relationship. The segment on the “many” side becomes the child, and the segment on the one side becomes the parent.

• The following rules:

Transforming Many-Many Relationships

A more complex situation

PRODUCT

SUPPLIER

IS-SUPPLIED-

BY

N

ID DESCRIPTION PRICE

NAME

LOCATION

M

ID DESCRIPTION PRICEPRODUCT

NAME LOCATIONMANUFACTURER

ID DESCRIPTION PRICEPRODUCT

NAME LOCATIONMANUFACTURER

3. For entity sets, E1 and E2, that have a many-many binary relationship, and from which segments S1 and S2 have been identified, construct two different one-many PCRs: S1 to S2, and S2 to S1.

Many-Many Relationship with an Attribute

PRODUCT

SUPPLIER

IS-SUPPLIED-

BY

N

ID DESCRIPTION PRICE

NAME

LOCATION

M

QUANTITY

4. If a binary many-many relationship has attribute data, create a new intersection segment I, which contains that data.

ID DESCRIPTION PRICEPRODUCT

NAME LOCATIONMANUFACTURER

QUANTITYQUANTITY

ID DESCRIPTION PRICEPRODUCT

NAME LOCATIONMANUFACTURER

QUANTITYQUANTITY

The IMS Architecture

• Hierarchical data model has no standard

• Must be hierarchically oriented

• Limit on segment types

• Database Description (DBD)

• Program Specification Block (PSB)

• Program Communication Block

• DL/1

Defining the Physical Database – The DBD

• An example – four segment types– DEPARTMENT

– EMPLOYEE

– SKILL

– ASSIGNEDTO

• The DBD is analogous to the network schema

1 DBD NAME = DEPTPERS, ACCESS = HISAM

2 SEGM NAME = DEPARTMENT, PARENT = 0, BYTES = 20

3 FIELD NAME = (DEPTNAME,SEQ,U), BYTES = 10, START = 1, TYPE = C

4 FIELD NAME = MANAGER, BYTES = 10, START = 11, TYPE = C

5 SEGM NAME = EMPLOYEE, PARENT = DEPARTMENT, BYTES = 22

6 FIELD NAME = (EMPNAME,SEQ), BYTES = 20, START = 1, TYPE = C

7 FIELD NAME = YRS-SVC, BYTES = 2, START = 21, TYPE = P

8 SEGM NAME = SKILL, PARENT = EMPLOYEE, BYTES = 17

9 FIELD NAME = (SKILLCODE,SEQ), BYTES = 2, START = 1, TYPE = P

10 FIELD NAME = SKILLNAME, BYTES = 15, START = 3, TYPE = C

11 SEGM NAME = ASSIGNEDTO, PARENT = EMPLOYEE, BYTES = 4

12 FIELD NAME = (PROJNI,SEQ), BYTES = 2, START = 1, TYPE = P

13 FIELD NAME = DIVISION, BYTES = 2, START = 3, TYPE = C

14 DBGEN

Defining the Logical Database – The PSB

• The PCB

• Sensitive segments

• Analogous to the network subschema

• PROCOPTG = GET

I = INSERT

R = REPLACE

D = DELETE

A = ALL

1 PCB TYPE = DB, DBNAME = DEPTERS, PROCOPT = A, KEYLEN = 18

2 SENSEG NAME = DEPARTMENT, PARENT = 0

3 SENSEG NAME = DEPTNAME, START = 1

4 SENSEG NAME = EMPLOYEE, PARENT = DEPARTMENT

5 SENFLD NAME = EMPNAME, START = 1

IMS Access Methods

• HISAM

• HDAM

• HIDAM

• HSAM

DEPTNAME

Smith

MANAGER

Marketing

Bond

EMPNAME

7

YRS-SVC

Hill

EMPNAME

5

YRS-SVC

Programmer2

SKILLCODE SKILLNAME

SystemsAnalyst

5

SKILLCODE SKILLNAME

Sales10

SKILLCODE SKILLNAME

Tree Occurrence Example

Hierarchical Pointers

DEPTNAME

Smith

MANAGER

Marketing*

Bond

EMPNAME

7

YRS-SVC

*

Programmer2

SKILLCODE SKILLNAME

*SystemsAnalyst

5*

Hill 5*

Sales10o

Child and Twin Pointers

DEPTNAME

Smith

MANAGER

Marketing**

(TwinPointer)

YRS-SVC

Bond

EMPNAME

7**

(ChildPointer)

SKILLNAME

Programmer2

SKILLCODE

*o

Hill 5o*

SystemsAnalyst

5oo Sales10oo

The IMS Data Manipulation Language

• Program Work Area

– Segment templates

– Currency templates

• The language?

GET UNIQUE (GU)

GET NEXT (GN)

GET NEXT WITHIN PARENT (GNP)

<hold options> GHU, GHN, GHNP

INSERT (ISRT)

REPLACE (REPL)

DELETE (DLET)

Retrieve the first segment that satisfies a given condition

Retrieve the next segment

Retrieves the next segment, but only within the current parent

Lock the database for GU, GN, GNP

Add a new segment to the database

Modify the value of a segment field

Delete a segment

DL/1

• Data Language 1• Commonly used commands

Command <segment name> <WHERE qualification>

1. A Simple Segment Retrieval

GU DEPARTMENT (DEPTNAME = ‘Marketing’)

• Get Unique (GU)

2. Retrieval of a Dependent Segment

GU DEPARTMENT (DEPTNAME = ‘Marketing’)

EMPLOYEE? (EMPNAME = ‘Steve Smith’)

3. Retrieval of dependent Segment When Parent is Not Known

GU DEPARTMENT

EMPLOYEE (EMPNAME = ‘Steve Smith’)

4. Retrieval of a Set of Segments

GU DEPARTMENT (DEPTNAME = ‘Marketing’)

EMPLOYEE

GN EMPLOYEE

• Get Next (GN)

5. Retrieving All Segments of a Particular Type

GU DEPARTMENT

EMPLOYEE

MORE GN EMPLOYEE

GOTO MORE

6. Retrieving Segments for Just One ParentGU DEPARTMENT (DEPTNAME = ‘Marketing’)

EMPLOYEE

GNP EMPLOYEE

• Get Next Within Parent (GNP)

7. Retrieving Segments for Just One Parent

GU DEPARTMENT (DEPTNAME = ‘Marketing’)

EMPLOYEE (EMPNAME = ‘Steve Smith’)

SKILLS

NEXT GNP SKILLS

GOTO NEXT

• GET HOLD UNIQUE• GET HOLD NEXT• GET HOLD NEXT WITHIN PARENT

• Get Hold

• Replace (REPL)

8. Modifying Segment Field Values

GHU DEPARTMENT (DEPTNAME = ‘Marketing’)

EMPLOYEE (EMPNAME = ‘Irving Valtz’)

MOVE 55000 TO SALARY

REPL

• Delete (DLET)

9. Deleting a Segment

GHU DEPARTMENT (DEPTNAME = ‘Marketing’)

EMPLOYEE (EMPNAME = ‘Irving Valtz’)

DLET

• Insert (INSRT)

10. Adding a Segment

MOVE 598 TO SKILLCODE

MOVE ‘ENGINEERING DRAFTSMAN’ TO SKILLNAME

MOVE 0 TO YRS-EXPERIENCE

INSRT DEPARTMENT (DEPTNAME = ‘Engineering’)

EMPLOYEE (EMPNAME = ‘Bob Lee’)

SKILL

1. Data representation

Hierarchical Data Model Evaluation

2. Data manipulation language