Discussion
There is an equivalent join expression that can be substituted for all subquery expressions.
Answer: B
Correct answer: False
Some subqueries have no join equivalent. A correlated NOT EXISTS, or a comparison against an aggregate of a related set such as WHERE salary > (SELECT AVG(salary) FROM emp), cannot be expressed as a join.
The reason is structural: a join produces rows by combining them, whereas a subquery can return a single computed value or merely test whether any row exists. Many subqueries can be rewritten as joins, and doing so sometimes helps the optimiser - but the statement claims all of them can, and that is what makes it false.
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