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 in a subquery and whether the select list of that subquery should typically select all columns as a placeholder.
Option 1: True. The statement claims that the inner subquery often selects all columns as a placeholder because the actual column values don’t matter for EXISTS/NOT EXISTS. This is incorrect. The EXISTS/NOT EXISTS clause does not use any data from the outer query’s row, it only tests whether the subquery returns any rows. Therefore, selecting all columns is unnecessary and can hurt performance. In practice, you would typically write something like SELECT 1 or SELECT NULL (and you can even omit the select list entirely in some SQL dialects) to minimize work.
Option 2: False. This option correctly states that the idea of selecting all columns as a placeholder is not how EXISTS/NOT EXISTS is typically written. The inner query need only check for the existence of rows; it does not rely on the actual column values, so choosing a minimal constant expression (such as 1) suffices and is the conventional approach. It reflects an understanding of how EXISTS evaluates truth based on the presence of rows rather than their content.
In summary, EXISTS/NOT EXISTS focuses on row existence, not the data in the rows, so the inner subquery is usually written to return a minimal constant (like 1) rather than all columns; the stated premise in Option 1 mischaracterizes common practice, making it false and Option 2 true.

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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