The business partner code alone is not unique. For a business partner that has a contact person in each department, there are as many records with the same business partner code as there are departments, so it cannot be narrowed down to one record.
×Incorrectイ
Names of contact persons can be duplicated, so a name gives no guarantee as an identifier for business data. Moreover, because the structure places one contact person in each department, the column that essentially distinguishes the records is the department name; making the name part of the primary key is not appropriate.
○ウ
正解A primary key is the smallest combination of columns that can distinguish one record in a table from all the others and identify it uniquely. In this question the preamble states that a business partner has, in principle, one contact person, but depending on the business partner there may be one contact person per department. In other words, several contact-person records can share the same business partner code, one for each department. Therefore the business partner code alone is not enough, and the department name must be added as the column that tells those records apart. Conversely, different business partners have different business partner codes, and within the same business partner each department has exactly one contact person, so the combination of business partner code and department name always narrows the result down to a single record. The primary key is therefore business partner code plus department name, and ウ is correct.
×Incorrectエ
Because the business partner code is not included, a conflict occurs if a different business partner has a contact person with the same name and the same job title (for example, a sales manager named Tanaka). The column showing which business partner the record belongs to is missing, so uniqueness cannot be guaranteed.
ここがポイント
A primary key is the column, or the smallest combination of columns, that uniquely identifies a record. When one column is not enough, combine several columns into a composite key.
A primary key is decided not only by the table definition but also by the business rules given in the preamble, such as whether there is one contact person per business partner or one per department. The trick is to find the unit at which duplicates can occur and add the column that distinguishes it.