Subqueries vs. joins: Judge the generality of this claim.\n“There is an equivalent join expression that can be substituted for all subquery expressions.”

Introduction / Context:
SQL supports both subqueries (nested SELECT statements) and joins to combine and filter data. In many cases, the optimizer can transform one into the other. However, believing that every subquery has a join-equivalent is an overgeneralization. This question probes that nuance.

Given Data / Assumptions:

• Consider both correlated and non-correlated subqueries.
• Consider predicates involving EXISTS, NOT EXISTS, IN, NOT IN, and scalar subqueries.
• Standard SQL null-handling applies.

Concept / Approach:
Many non-correlated IN/EXISTS subqueries can be rewritten as joins (for example, semi-joins) to achieve equivalent results. However, NOT IN and NOT EXISTS with NULL-sensitive semantics, as well as certain correlated scalar subqueries or complex conditional logic, may not map cleanly to straightforward inner/outer joins without changing semantics—especially in the presence of NULLs or when duplicates matter. Even when a theoretical relational algebra equivalence exists (for example, anti-join vs. NOT EXISTS), rewriting requires care to preserve behavior and performance, and some constructs are naturally clearer as subqueries.

Step-by-Step Solution:

Verification / Alternative check:
Test tricky cases with NULLs and duplicates; results can diverge between a naive join rewrite and the original subquery behavior.

Why Other Options Are Wrong:

• “Correct” is too absolute.
• “Only for non-correlated” is closer but still not universal; edge cases exist.
• Null handling and optimizer choices do not guarantee join equivalence.

Common Pitfalls:
Rewriting NOT IN to a left join filter without handling NULLs; changing cardinalities by accidental duplication during joins; losing performance despite semantic equivalence.

Final Answer:
Incorrect