| # Introduction to Entity Relationship Modeling Entity Relationship Modeling gives an insight to the LN application databases and the way in which they are interrelated. Entity Relationship Modeling is composed of two main building blocks: Entity types Entity relationships These building blocks are interrelated and used in entity relationship diagrams to show the relationships between the permanent storage components. Together with the other components listed below, they let you illustrate the diversity of relationships between the different databases. A group of related diagrams make up an entity relationship model. Entity type A person, place, thing, or concept that you want to record information about. An entity type is a group of entities with common attributes and can be part of a diagram, such as `Trucks`. Entity A single occurrence of an entity type; a fact relevant to the company, and about which information is permanently stored. For example: `Truck-A` and `Truck-B`. Three kinds of entities exist: Logical entity Entities that have a meaning to the real world and are comprised of one or more physical entities; they are defined on a higher abstraction level. Physical entity Database tables of the LN application. Associative entity An entity used to link other entities. An associative entity type is only used when there is a many-to-many relationship between two entity types. Beispiel There could be an M:N relationship between `EMPLOYEE` and `TRUCK` regarding maintenance. This relationship does not show which `EMPLOYEE` maintains which `TRUCK`. There are also other attributes to be considered, such as time spent, part no. and so on. As a result of the ambiguity regarding the Many-to-many relationship, the associative entity type `MAINTENANCE` can be defined. (See figure 2). Associative entity type Attribute A fact or non-decomposable piece of information describing an entity type; for example, Number of wheels. Attribute value The value of an attribute; for example, 4 wheels. Relationship A reason of relevance for associating entities of one or two entity types. Cardinality A specification of the number of possible entities for each entity type of a pairing. The three types of cardinality are as follows: One-to-One (1:1) A one-to-one relationship. Only one instance of entity type B can be associated with only one instance of entity type A. One-to-Many (1:N) A one-to-many relationship. Multiple instances of entity type B can be associated with only one instance of entity type A. Many-to-Many (M:N) A many-to-many relationship. Multiple instances of entity type B can be associated with multiple instances of entity type A and vice-versa. Within Entity Relationship Modeling, there is no real difference between logical and physical entities. The Entity Relationship Modeling procedure There are two different approaches for modeling databases: The first is the top down approach, which has to be used when physical entity types, entity relationships and entity relationship diagrams have not yet been created. Therefore, you have to manually create and define all entity types and relationships. The second is reverse engineering, which is a quick way to model entity relationship diagrams because you can select existing database/table definitions for which a diagram will be created automatically. Reverse engineering can only be used in the case of LN. Carrying out the Entity Relationship Modeling procedure results in a model that shows the relations between different databases. Figure 14.2 shows the steps in the Entity Relationship Modeling top down procedure. The Entity Relationship Modeling top-down procedure Entity Types (tgerm2500m000) To create the entity types, use the Entity Types (tgerm2500m000). To define an entity type, complete the following steps: In the Entity Types (tgerm2500m000) session, click New In the Entity Type field, specify an id for the entity type. Click Save
Key fields for the Entity Type Details (tgerm2100s000) session
Field | Description
--- | ---
Owner Package | The package to which the entity type is related.
Owner Module | The module to which the entity type is related.
Link to Diagram | The entity-relationship diagram into which an entity type is decomposed. At a different level in the DEM-Tool, you can see the entity-type relationship between the current entity type and other entity types.
Entity Type Category | If you select this check box, the entity type is a physical entity type or a logical entity type related to the Baan package.
Associative Entity Type | If you select this check box, the entity type serves as a link between two other entity types that have a many-to-many relationship between them.
Tables by Entity Type (tgerm4100m000) To select one or more tables from the Table Definitions (ttadv4526m000) session, and to link those tables to the selected entity type, use the Tables by Entity Type (tgerm4100m000) session. To start this session: On the Zusatzoptionen menu of the Entity Types (tgerm2500m000) session, click Link ERP Tables to Entity Type... One entity type can be linked to several tables. The interdependent relations between these tables are shown at different levels in the Modeler. Entity Type Relationships (tgerm3500m000) To list and define entity type relationships, use the Entity Type Relationships (tgerm3500m000) session. The relationships listed in the Entity Type Relationships (tgerm3500m000) session have been established between the entity types listed in the Entity Types (tgerm2500m000) session. To define entity type relationships, in the Entity Type Relationship Details (tgerm3100s000) session, select the entity types between which relationships are to be established. You can also define the cardinality of the relationship between the selected entity types. In the case of an M:N relationship, an associative entity type can be created, and a table can be selected from the Table Definitions (ttadv4526m000) session to serve as a link between two entity types. To define a relationship between two entity types, complete the following steps: In the Data Entity Relationship field, specify an ID for the relationship. In the From Entity Type field, select the first entity type involved in the relationship. In the To Entity Type field, select the other entity type involved in the relationship. In the Min Cardinality From - To field, specify the number of possible entities for each entity type of a pairing for the entity type you selected in the From Entity Type field. In the Min Cardinality To - From field, for the entity type you have selected in the To Entity Type field, specify the number of possible entities per entity type of a pairing. Click Save
Key fields for the Entity Type Relationship Details (tgerm3100s000) session
Field | Description
--- | ---
Relationship is Subtype | Wenn dieses Kontrollkästchen markiert ist,, the entity-type relationship is between a subtype and a supertype entity type; it is used to indicate that the supertype’s attribute also applies to the subtype.
Physical Database Relation exists | Wenn dieses Kontrollkästchen markiert ist,, the entity-type relationship is between entity types that have tables from the LN data dictionary linked to them.
Associative Table | An LN table used to link two entity types that have a many to many relationship between them
Infinite | Wenn dieses Kontrollkästchen markiert ist,, the maximum to-from relationship is infinite.
Entity Relationship Diagrams (tgerm1500m000) To create and maintain the entity-relationship diagrams, use the Entity Relationship Diagrams (tgerm1500m000) session. An entity-relationship diagram is a graphical design of the relational data model structure. The diagram shows a multilevel structure that consists of entity types and entity-type relationships. To create an entity-type relationship diagram, complete the following steps: In the Entity Rel. Diagram field, specify an ID for the entity relationship diagram. In the Owner field, select the LN user who created or is responsible for the entity relationship diagram. Select who is authorized to modify the diagram by selecting one of the check boxes under To be modified by. Click Save |