Question at position 37 When EXISTS or NOT EXISTS is used in a subquery, the select list of the subquery will usually just select all columns as a placeholder because it doesn't matter which columns are returned.TrueFalse

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BlackTom AI · Accepted Answer

When evaluating EXISTS or NOT EXISTS in SQL, the inner subquery is used only to determine whether any rows exist, not to return data to the outer query.
Option A: True. This statement aligns with standard practice: the select list inside an EXISTS subquery is effectively irrelevant to semantics, so developers often write SELECT 1 (or SELECT 0) or otherwise avoid pulling actual columns. The engine only cares about existence, not the returned column values.
Option B: False. It would be inaccurate to claim that you must or should routinely select all columns as a placeholder. Returning all columns (SELECT *) is wasteful and unnecessary for an EXISTS check, since the outer query does not use any of those column values. The typical pattern is to select a constant (like 1) or use SELECT 1 AS dummy, which communicates intent and can improve readability and performance.
Option C (implicit in the prompt): If someone mistakenly thinks existence checks rely on the particular columns chosen, they’d be mistaken. The existence result is a boolean: does at least one matching row exist? It does not depend on the column list of the inner select, only on whether rows are returned.
Option D (also addressing performance): In some databases, the optimizer may simplify or rewrite the subquery, but the critical point remains: the existence decision does not depend on returning real data, so selecting all columns is unnecessary and not the usual approach.

Question at position 37 When EXISTS or NOT EXISTS is used in a subquery, the select list of the subquery will usually just select all columns as a placeholder because it doesn't matter which columns are returned.TrueFalse

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