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Entity Relationship Diagram

Entity relationship diagrams express the logical structure of a database the main components of entity relationship diagram are :

Symbol Meaning

  1. 556.png Rectangle Entity set
  2. 561.png Ellipse Attributes
  3. 566.png Diamonds Relatiohship stes
  4. 571.png Lines Linkd attributes to entity sets and entity sets to relationship sets
  5. 576.png Double ellipse Multivalued attributes
  6. 581.pngDotted ellipse Derived attributes
  7. 586.pngDouble line Total participation of entity in relationship sets
  8. 591.pngWeak entity
  9. 596.pngIdentifying relationship for weak entity set

Translating Tables into ERD

  1. The following rules can be followed to convert tables into ERD
  2. Create a table for all strong entities and column for all attributes. If an attribute is a composite attribute then create columns only for sub attributes.
  3. Do not create any column for derived attributes as they would not be included in the table.
  4. Create separate table for multivalued attribute and the primary key of the entity would be added as a column to the table of the multivalued attribute.
  5. Similarly create separate table for weak entity and the primary key of weak entities/multivalued attributes is formed by combining partial key of weak entity and primary key of strong entity to which the weak entity is related to.
  6. Tables for relations depend upon the cardinality of the relationship. For one to many and one to one there is no need to create separate tables.
  7. For many to many, create a separate table for relationship and add primary key by combining the primary key of both the entities.

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