E-R Structures:

The E-R model is based on various structures. These structures are formed out of the entities and they define the relationships between those entities. Refer to Figure 8.8 for the various structures.

Following are some of the E-R structures:

Classic 1 to many structure
The Classic 1 to Many structure is also called the Master/Detail or Parent/Child structure. Purchase Order/Items, Customer Order/Items, Employee/Dependants, etc. are examples of such structures.

Tree structure

The tree structure is also referred to as a hierarchy. A tree structure is a series of 1:N relationships. In this structure, each node has only one parent and each parent has zero, one, or many children. The children with a common parent are called siblings. For example, a company is a parent which consists of many departments. All departments are part of a single entity called the company. They are together called siblings. For converting a tree structure into a relationship, steps similar to the ones used for converting 1:N relationships should be followed and should be repeated for each of them.

Network structure

Network structures are of two types: simple and complex.
A simple network structure consists of 1:N relationships. But in this structure, each node can have more than one parent. For converting a simple structure into a relationship, similar steps which were used for converting 1:N relationships should be followed. The relation would have two foreign keys as the entity has two parents.
A complex network structure has only one M:N relationship. For converting a complex network into a relation, steps similar to those used for converting simple networks should be followed. An intersection relation should also be introduced during the conversion.

Importance of the E-R Model

Since the year 1976, the E-R model has undergone a significant transformation. The model has been extended to and is being widely used in the field of database designing. For a database designer, the E-R model:
Goes well with the relational model. The constructs used in the model (entities and relationships) can be transformed easily into relational tables.

• It is simple and can be easily understood by the end user.
• It requires minimal training.
• It can be used as a design plan for implementing a data model in database management software.

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