Keys and identifiers: what is a compound key, with respect to how items are uniquely identified in a table?

Introduction / Context:
Primary keys ensure that each row in a table is uniquely identifiable. Sometimes no single column can serve as a unique identifier, so designers combine more than one column to achieve uniqueness. This concept is foundational for normalization and referential integrity.

Given Data / Assumptions:

• We are defining “compound key” (also called composite key).
• Goal: uniqueness of a row by combining multiple attributes.
• Examples include (country_code, phone_number) or (order_id, line_number).

Concept / Approach:
A compound key is formed by two or more fields whose combined values are unique, even if the individual fields are not unique by themselves. It therefore is “made up of several pieces of information” and “uniquely identifies an item,” satisfying both descriptions in the options.

Step-by-Step Solution:

Verification / Alternative check:
In SQL, PRIMARY KEY (col1, col2) defines a composite primary key; uniqueness is guaranteed across the combined tuple values.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing composite keys with concatenated string identifiers; or thinking each participating column must be unique individually (they need not be).

Final Answer:
both (a) and (b)