Question at position 54 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

The question asks about the behavior of EXISTS or NOT EXISTS when used in a subquery. This is a common topic in SQL semantics.
Option 1: True. The statement claims that 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. In actuality, for EXISTS and NOT EXISTS, the subquery is evaluated for the existence of any row that satisfies the predicate, and the values of the selected columns inside the subquery are irrelevant. Therefore, many examples use SELECT * or SELECT 1 as a placeholder, since the engine only checks for the presence of rows, not their content. This makes the statement essentially correct in spirit, though some dialects might have syntax preferences like SELECT 1 or EXISTS(SELECT 1 ...).
Option 2: False. This would imply that the select list must be specific or that the actual column values influence the outcome of EXISTS/NOT EXISTS. That is inaccurate because EXISTS/NOT EXISTS returns a boolean based on whether the subquery yields at least one row; the content of the selected columns is not used in determining the boolean result. The only thing that matters is whether a row exists, not which columns are selected.
In summary, the core idea is that for EXISTS/NOT EXISTS the subquery is evaluated for the existence of rows, and the particular columns returned are not used in the truth evaluation, which aligns with the general practice of using a placeholder like SELECT 1 in such subqueries.

Question at position 54 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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