Software has the following characteristics.
① Software is a logical entity with abstraction.
② Software does not have a clear manufacturing process.
③ Software does not experience wear and aging issues during its usage.
④ Software is dependent on hardware and the environment.
⑤ Software is highly complex and expensive.
⑥ Software development involves many social factors.
Nature of relationships:
① The finiteness of tuples: The number of tuples in a two-dimensional table is finite.
② The uniqueness of tuples: No two tuples in a two-dimensional table can be completely identical.
③ The irrelevance of tuple order: The order of tuples in a two-dimensional table, i.e., the order of rows, can be arbitrarily exchanged.
④ The atomicity of tuple components: The components of tuples in a two-dimensional table are indivisible basic data items.
⑤ The uniqueness of attribute names: Different attributes in a two-dimensional table must have different attribute names.
⑥ The irrelevance of attribute order: The order of attributes in a two-dimensional table can be arbitrarily exchanged.
⑦ The homogeneity of component value domains: The components of attributes in a two-dimensional table have the same value domain as that attribute, or in other words, the columns are homogeneous. A two-dimensional table that satisfies the above seven properties is called a relation, and the model established with the two-dimensional table as the basic structure is called a relational model.
Data Flow Diagram:
A data flow diagram is a graphical representation of the system’s logical model, depicting the movement and transformation of data from input to output from the perspective of data transmission and processing. It directly supports the functional modeling of the system.