Data Models: Hierarchical Model

Hierarchical model is based on tree structure. A hierarchical database consists of collection of records,  that  are connected  to  each  other by  links.

Record : A record  is a  collection  of attributes,  each contains  only one  data value.

Link : A link  is an association  between two records.

The tree structure used in hierarchical model is known as rooted tree. The Root node of that tree is dummy node or an empty node. So, hierarchical model is a collection of rooted trees. Collection of rooted trees make forest. A rooted tree is also known as database tree.

1. Tree Structure Diagrams

Tree structure consists of two basic components:

  1. Rectangular boxes : Rectangular boxes represent various record types.
  2. Line : Line represents link between two record types.

A tree structure diagram specifies the overall logical structure of database (as E-R diagram in Entity Relationship model).

Relationship exists between a parent and a child must be one-to-many or one-to-one relationship.

Link between parent and child is represented by line with an arrow. A parent may have an arrow pointing  to child,  but child  must have  an arrow  pointing to  its parent.

A general tree structure is shown in Figure 4.1.

Example. Consider  the  relation  working-for between  Employee  and  Department

Record Employee consists of three attributes (Name, Age and Salary) and record Department consists of two attributes (Dept-ID and Dept-Name). An E-R diagram for this relation is shown in  Figure  4.2.

  • First, suppose an employee can work in only one department but any department can have more  than  one employee. Corresponding tree structure diagram is shown in Figure 4.3(a) and sample database in Figure  4.3(b).

  • Now suppose an employee can work only in one department as well as every department can have only one employee.

Corresponding tree structure diagram is shown in Figure 4.4(a) and sample database in Figure  4.4(b).

  • Now suppose an employee can work in more than one department and any department can have more than one employee. In that case we have to make two separate tree structure diagrams because Many-to-Many relationship is not allowed in tree structure diagram.

Corresponding tree structure diagrams are shown in Figure 4.5(a), (b).

Corresponding sample database for tree structure diagrams in Figure 4.5(a), (b) are shown in Figure  4.6(a), (b) respectively.

In this case, the sample database consists of more than one tree structure diagram.

  • In case of descriptive attributes, tree structure diagram is more Consider the E-R diagram shown  in  Figure  4.7.

Because a link cannot have any value, so we have to make new record for descriptive attribute. Corresponding tree structure diagrams for E-R diagram in Figure 4.7 are shown in  Figure  4.8(a),  (b). Assume  conditions  of  4.2.1 (iii).

Corresponding sample database for tree structure diagrams in Figure 4.8(a), (b) are shown in Figure  4.9(a), (b) respectively.

2. Operations on Hierarchical Data Model

The basic operations that can be performed on Hierarchical data model are insertion, deletion, updation and retrieval. All these operations  discussed briefly.

  1. Insertion Operation : The Insert Operation is used to insert a new record into the The newly inserted record becomes the current record for the database.

If the Inserted Record is a root record then it creates new hierarchical tree with the new record as the root. But if it is a child record then we should make its parent first because  a child  node  cannot exist  without a  parent  (root).

  1. Deletion Operation : The delete operation is used to delete a record from the To delete a record from the database, we first make it the current record and then issue the delete command.
  2. Updation Operation : The updation operation is used to update a record in the database.
  3. Retrieval Operation : The process of searching and fetching of a record in the database is known  as retrieval  of  a record.

3. Query Language for Hierarchical Databases

Consider the  example of  Employee—Department  relation schema.

Program Work Area

Program work area is a buffer storage area which contains the following variables.

  • Record template : Record template is a record for each record type for example Employee record for Employee record type, Department record for department record type.
  • Currency pointers : It is a set of pointers, one for each database tree which contains the address of most  recently  used record.
  • Status flag : It is a variable which represents the result of most recent database operation.

Get Command

Get Command  is  used for  data  retrieval.  The general  format  of  Get command  is

There are two types of Get command. These are:

  • Get first : This command gives the first record of a given record type from the database which satisfies the given The general format of Get first command is

Ex.                                     Get first <Employee>

Where employee. Dept–name = “Developing”;

gives employee Punit.

  • Get next : This command gives the next record of a given record type from the database which satisfies the given The general format of Get next command is

Ex.                                    Get next <Employee>

Where employee. Dept-name = “Developing”;

gives employee Sunil.

If you want to locate a  record within the same parent  then use the command

Update Commands

  • Insert command : This command is used to insert a new record of a given record type into the The general format of insert command is

Ex. For adding a new employee we write the query = “Rahul”;

employee.age = “25”;

employee.salary = “8000”;

Insert employee

Where department.Dept-Id = ”3A”;

  • Replace command : This command is used to modify an existing record in database. The general format of replace command is

Ex.                          get hold first <employee>

where employee.Name = “Sam”;

employee.salary = “9000”;


  • Delete command : This command is used to delete an existing record in The general format of delete command is

Ex.                          get hold first <employee>

where employee.Name = “Sunil”;


Delete command not  only deletes  a parent but  also deletes  all of  its children.

4. Virtual Records

In tree structure diagram, you cannot represent many-to-many relationship directly. To represent these relations and keep tree-structure organization you have to replicate data. To overcome the drawbacks  of data  replication virtual  records  are used.

A Virtual record is a record with no data values but it contains a pointer to the physical record. To avoid replication, keep a single record and place virtual record instead of actual record. Corresponding tree structure diagram with virtual records of Figure 4.9(b) is shown in Figure  4.10.

Virtual records keep data consistency but wastage of storage space is still a serious problem.

5. Implementation of Tree Structure Diagram

To optimize tree structure diagram, leftmost–child, preorder threads and next-sibling pointers are used instead of parent-child pointers. Consider Figure 4.6(a), the corresponding optimized tree structure diagram  is shown  in Figure  4.11.

Advantages of Hierarchical Model

The following are the main advantages  of hierarchical data model:

  1. Simplicity : In this model, records are related in form of parent/child relationship. So performing various operations (e., insertion, deletion etc.) in this tree like structure is easy and simple to perform. This results in the simple design of the database resulting from this  model.
  2. Integrity of Data : The parent/child relationship between the various records in the hierarchical model is represented by a relationship or Each child segment can be linked to only one parent and a child can only be reached through it parent, so this model promotes data integrity.
  3. Data Security : Each child segment can be linked to only one parent and a child can only be reached through its parent in this So for deleting the child segment proper information of parent segment is needed. Thus it provides data security which is enforced by  the  DBMS.
  4. Efficiency : The hierarchical model contains one to many relationships between parent and When the database contains many 1 : N relationships between various records then this model handles it very efficiently.
  5. It is very efficient to handle large number of transactions using this This is mainly because the links (or relationship) established by the pointer in the various records are permanent and  cannot  be modified.

Disadvantages of Hierarchical Model

The information is replicated in hierarchical database. The replication may occur either in different database trees or in same tree. Consider Figure 4.9(b), where records (1A, developing) and (2A,  testing) are  replicated. The  other  disadvantages are  as follows:

  1. Knowledge of physical level of data storage is required : The requirement for a one to many relationship between parent and child can result in redundancy of data.

To get around the redundancy problems, data is stored in one place and referenced by links  or  physical pointers,  which  requires  technical skills.

  1. Complexity : The physical links make it very difficult to expand or modify the database, changes typically require substantial rewriting efforts.
  2. Inflexibility : The basic problem occurs with this model is that they are not flexible enough to establish all the relationships (many-to-many etc.) which occur in the real Usually there are one to many relationship between the records, established by pointers which are permanent and cannot be modified in case of other cases where relationships (like  many  to many  etc.)  exist.
  3. Lack of querying facilities : The lack of declarative querying facilities and need for navigation of pointers to access needed information make querying rather It does not provide the adhoc query capability  easily.
  4. Database management problems : In this model, the modifications to the data structure leads to significant modifications to application programs that access the Also new relations or nodes result  in complex system  management tasks.
  5. Problems with data manipulation operations : Various problems are encountered while performing various operations like insertion, deletion and Moreover the data retrieval is also very complex and asymmetric. Therefore, a better model is needed to solve  these  problems.
  6. Lack of standards : This model does not have any specific or precise standard for database design and modelling.

Note: Record-type is  equivalent  to  name of  table  in  relational model  and  entity  set in  E-R  model.

Record is equivalent to tuple in relational model and entity in E-R model. But generally  we  use term  record  instead of  record-type.

This is  applicable for  both  hierarchical model  and  network model. 

Commercially Available Hierarchical Database Systems

There are number of Database systems based on hierarchical model. Some of them are as follows :

  1. IBM’s Information Management System.
  2. MRI’s System 2000
  3. IMS Informatics Mark IV.
  4. Time-shared Data Management System (TDMS) of SDC.

Source: Gupta Satinder Bal, Mittal Aditya (2017), Introduction to Database Management System, 2nd Edition-University Science Press (2017)

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