SQL query semantics: Which command correctly returns unique subject values (SUB) from a table named BOOK, eliminating duplicates in the result set?

Introduction / Context:
SQL provides the DISTINCT keyword to remove duplicate rows from a projection. When you need a list of unique values from a column, DISTINCT is the standard, portable approach across vendors.

Given Data / Assumptions:

• Column name is SUB and table name is BOOK.
• We require uniqueness in the output.
• We assume conventional SQL syntax.

Concept / Approach:
SELECT DISTINCT column_list returns each unique combination of the listed columns exactly once. SELECT column without DISTINCT returns all rows (including duplicates). SELECT ALL is the default behavior (it does not de-duplicate). Therefore, to obtain unique subject values, we must use DISTINCT with the specific column.

Step-by-Step Solution:
Identify the target: unique values of SUB.Choose the SQL construct for uniqueness: DISTINCT.Write the minimal projection: SELECT DISTINCT SUB FROM BOOK.

Verification / Alternative check:
Running SELECT SUB FROM BOOK ORDER BY SUB would still show duplicates; wrapping with DISTINCT removes them. Some systems also support GROUP BY SUB to get unique SUB values, but DISTINCT is simpler and clearer.

Why Other Options Are Wrong:

• SELECT ALL FROM BOOK: Returns all columns and all rows; includes duplicates.
• SELECT SUB FROM BOOK: Returns all SUB values with duplicates.
• All of the above: Incorrect because only one option eliminates duplicates.

Common Pitfalls:
Using DISTINCT with multiple columns inadvertently groups by the entire combination; ensure only necessary columns are projected when deduplication is intended.

Final Answer:
SELECT DISTINCT SUB FROM BOOK