Structuring Database for Accounting

type students has unique values through which a student instance can be identified. Similarly, Code is a key attribute of entity type Accounts because its data values are required to be unique. Fig. .

: Diagrammatic presentation of an entity type accounts with code as key attribute Some times two or more such attribute together (called composite key) may constitute such distinct values. For example, the student entity type that has entity instances across several sections of a class in a school shall require a composite key of attributes (Sections and RollNo). But in any case, it is a constraint that does not allow any two-entity instances from having the same value for the key attribute at a point of time. Some entities may have more than one Key attribute.

The entity types, which do not have a key attribute at all are called weak entities. Relationship among two or more entity types represents an interaction among their respective entities. Whenever an attribute (say Debit) of one entity type (say vouchers) refers to another entity type (say Accounts), there exists a relationship between these entities (Vouchers and Account). For example, vouchers and accounts are related in two ways: vouchers contain