IPA IP 2023 Question 78

Descriptions A and D — a primary key must identify every record uniquely, and it may be made of more than one field.

Each description in turn:

• •A, correct — the primary key is what tells one record from another, so a field whose values repeat across records cannot be the primary key (nor can one that may be empty, NULL)
• •B, wrong — a primary key and an index can be set on the same field; most database systems in fact create an index on the primary key automatically to enforce its uniqueness and speed up searches by it
• •C, wrong — the data type does not matter, only uniqueness: codes such as G001 or text such as an e-mail address serve as well as numbers
• •D, correct — when no single field is unique, a combination can be: in a grade table, student number plus subject code identifies a row (a composite key)

Answer (b)

Why not others:

• •(a) — includes C, but a primary key need not be numeric
• •(c) — B and C are both wrong, and the two correct descriptions are left out
• •(d) — includes B, but a primary key is normally indexed, not barred from an index

Key rule: A primary key needs unique, non-null values; its data type is free, and it may combine several fields into a composite key.

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