Explain the conventions for displaying an ER schema as an ER diagram?
An E-R Diagram scheme might describe certain constraints to that the contents of a database must conform.
Mapping Cardinalities: Express the number of entities to that another entity can be related through a relationship. For binary relationship sets among entity sets A and B, the mapping cardinality must be one of:
1. One-to-one: An entity in A is related along with at most one entity in B, and an entity in B is related with at most one entity in A.
2. One-to-many: An entity in A is related along with any number in B. An entity in B is related with at most one entity in A.
3. Many-to-one: An entity in A is related with at most one entity in B. An entity in B is associated with any number in A.
4. Many-to-many: Entities in A and B are related along with any number from each other.
The suitable mapping cardinality for a particular relationship set depends on the real world being modeled. Cardinality ratios for binary relationships are represented on ER diagram through displaying 1,M, N etc. on the diamonds.
Existence Dependencies: if the existence of entity X depends on the existence of entity Y, then X is said to be existence dependent on Y.