問76
頻出度★★★
E-R図の説明と, その応用例として, 適切なものはどれか。
平成24年度 春期 ITパスポート試験 問76
テクノロジ系データベース › データベース設計
This question's explanation and your answers so far.
解説
問 76
Incorrectア
‘A diagram representing the order and duration of tasks’ used for ‘managing the schedule of system development’ describes an arrow diagram (PERT chart). Arrows represent tasks and the time they take, nodes represent the precedence among tasks, and the critical path (the longest path) is computed to manage the total number of days required and the available slack. It handles a schedule rather than the structure of data, so it is not an E-R diagram.
- イ
正解 The correct answer is イ. An E-R diagram (Entity-Relationship Diagram) is a data-modeling notation that expresses the things handled in a business as ‘entities’ and the connections between entities as ‘relationships’. The first half of choice イ, ‘a diagram representing the relationships among entities’, is exactly this definition. The application example in its second half is correct as well: an E-R diagram is used in the design process of a relational database, where entities are turned into ‘tables’ and relationships are turned into correspondences between tables expressed by foreign keys. Therefore ‘used to represent the relationships among tables in a relational database’ is a typical use of an E-R diagram, so イ, which is appropriate in both its description and its application example, is the answer. In addition, it is common to annotate a relationship with a multiplicity (cardinality) such as one-to-one, one-to-many, or many-to-many, and a many-to-many relationship is broken down into an intermediate table during design.
Incorrectウ
‘A diagram that combines sequence, selection, and iteration structures’ used to ‘describe the flow of a program’ describes a flowchart or a structured chart. It writes out processing steps by combining the three basic structures of structured programming, and its subject is the control flow of processing. What an E-R diagram represents is data (entities) and their relationships, not processing steps.
Incorrectエ
‘A diagram describing state transitions and conditions’ used to ‘describe the specification of a communication protocol’ describes a state transition diagram (statechart). It shows the states a system can take and, for each event or condition, which state it moves to, and it is used to define the behavior of communication protocols or embedded devices. It deals with ‘states’ that change over time, so its purpose differs from that of an E-R diagram, which represents the static relationships among entities.
ここがポイント
- An E-R diagram is a data-modeling notation that represents entities and the relationships among entities. In relational database design it is used to turn entities into tables and relationships into correspondences between tables.
- Learn to tell similar notations apart: an arrow diagram (PERT chart) manages schedules through task order and duration; a flowchart shows a processing flow made of sequence, selection, and iteration; a state transition diagram shows states and transition conditions (such as communication protocol specifications).
用語