Why does the query SELECT * FROM items WHERE price > ALL (SELECT price FROM items WHERE price return all rows if all prices are 100 or more?

hard📝 Conceptual Q10 of 15

PostgreSQL - Subqueries in PostgreSQL

Why does the query SELECT * FROM items WHERE price > ALL (SELECT price FROM items WHERE price < 100); return all rows if all prices are 100 or more?

ABecause ALL with an empty subquery always returns true

BBecause ALL with an empty subquery always returns false

CBecause price cannot be compared with ALL in this case

DBecause the subquery returns multiple columns

Step-by-Step Solution

Solution:

Step 1: Understand subquery result
The subquery selects prices less than 100. If none exist, subquery returns empty set.
Step 2: Behavior of ALL with empty set
In SQL, condition with ALL and empty set returns true (vacuous truth).
Step 3: Why query returns all rows
Since price > ALL (empty) is true for all rows, query returns all rows.
Final Answer:
Because ALL with an empty subquery always returns true -> Option A
Quick Check:
ALL with empty set = true (vacuous truth) [OK]

Quick Trick: ALL with empty subquery always returns true [OK]

Common Mistakes:

Thinking ALL with empty subquery returns false
Assuming subquery returns multiple columns
Confusing behavior of ANY and ALL with empty sets

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Why does the query SELECT * FROM items WHERE price > ALL (SELECT price FROM items WHERE price return all rows if all prices are 100 or more?

Step 1: Understand subquery result

Step 2: Behavior of ALL with empty set

Step 3: Why query returns all rows

Final Answer:

Quick Check:

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