This question's explanation and your answers so far.
社員数が 50 人で，部署が 10 ある会社が，関係データベースで社員や部署の情報を管理している。“社員”表と“部署”表の関係を示した E-R 図はどれか。ここで，1 人の社員が複数部署に所属することはない。下線のうち実線は主キーを，破線は外部キーを表す。E-R 図の表記は次のとおりとする。 〔表記法〕 [a]——[b] aとbが，1対1の関係であることを表す。 [a]——▶[b] aとbが，1対多の関係であることを表す。 [a]◀——[b] aとbが，多対1の関係であることを表す。 [a]◀——▶[b] aとbが，多対多の関係であることを表す。 社員
| 社員コード | 社員名 | 部署コード |
|---|
部署
| 部署コード | 部署名 |
|---|
（社員表：社員コードは実線下線（主キー），部署コードは破線下線（外部キー）。部署表：部署コードは実線下線（主キー）。）
[Employee]——[Department] is the notation for 1-to-1, which means that one dedicated department corresponds to each single employee. However, there are only 10 departments for 50 employees, so they cannot each be matched to a separate department. Besides, if the relationship really were 1-to-1, the two could simply be combined into a single table, which also does not fit the stated quantities of 50 employees and 10 departments.
[Employee]——▶[Department] is 1-to-many, that is, it reads as one employee corresponding to multiple departments. This runs squarely against the statement in the question that “no single employee belongs to more than one department.” It is a typical wrong answer that is easy to choose when the direction of the arrow is read backwards.
正解 Multiplicity is decided by counting from both directions. First, since “no single employee belongs to more than one department,” exactly one department corresponds to one employee. Conversely, there are 50 employees but only 10 departments, so multiple employees belong to one department. Therefore Employee:Department = many:1, that is, a “many-to-1” relationship. In the [Notation], [a]◀——[b] is defined as many-to-1, so putting Employee in place of a and Department in place of b gives [Employee]◀——[Department], which is choice ウ. The structure of the tables backs this up as well. Department Code in the Employee table has a dashed underline, meaning it is a foreign key, and it refers to Department Code, the primary key of the Department table. A foreign key is placed so that the table on the “many” side refers to the primary key on the “1” side, which agrees with reading Employee as the many side and Department as the 1 side.
[Employee]◀——▶[Department] is many-to-many, which means that multiple items correspond in both directions. It is true that, seen from the department side, there are multiple employees, but it also makes multiple departments correspond when seen from the employee side, and that violates the given condition. In addition, a many-to-many relationship cannot be expressed by giving the Employee table just one Department Code; a linking table (an associative entity) that joins the two is needed, so it does not fit the table structure shown either.