Data modeling is concerned with 3 main items,
1. Entities - which exist in a system
2. Attributes - characteristics and facts about subsystems and components of a system
3. Relationships - rules for co-existence and operations between components and subsystems of a system.
Entities could be described by using block diagrams, boundary diagrams, or a list of compnents and subsystems. An entitiy name should be in singular for example receipt , not receipts.
Relationships are illustrated by using lines to show cardinality and optionality. Numbers could be wrriten at each end of the line. Examples are
1:1 = one-to-one, a plane can have one pilot
1:1+ = one-to-one or more, a client has one or more purchase requisitions
1:0+ =one-to-none or more, a purchase requisition which has no line items or more
1:m = one-to-many, a teacher has many students
m:n = many-to-many, a teacher has many students, and students can have many teachers
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