Question at position 14 What is the requirement for using a subquery in the SELECT clause? the subquery must use an aggregate function. the subquery must return a single value. the subquery must return at least one value. the subquery must refer to the same table as the main query.

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BlackTom AI · Accepted Answer

When evaluating where a subquery is used in the SELECT clause, it’s important to recall the core requirement for scalar subqueries: they must return exactly one value for each row of the outer query.
Option 1: 'the subquery must use an aggregate function.' While aggregate functions are common in subqueries, they are not a universal requirement for a scalar subquery in the SELECT clause. A non-aggregate scalar subquery can also return exactly one value, so this statement is too strong and therefore incorrect.
Option 2: 'the subquery must return a single value.' This aligns with the standard rule for scalar subqueries in the SELECT list: the subquery must yield exactly one value (one row, one column) per outer row. If it returned more than one value, the query would fail with an error; if it returned zero values, it would typically yield NULL in the outer query. This option correctly captures the essential constraint.
Option 3: 'the subquery must return at least one value.' Saying 'at least one value' implies that it could return multiple values or zero values in some cases, which would violate the single-value requirement. A subquery in the SELECT clause cannot return more than one value, and returning zero values would lead to NULL rather than a concrete value. Therefore, this statement is inaccurate.
Option 4: 'the subquery must refer to the same table as the main query.' Subqueries in the SELECT clause are allowed to reference other tables or even be independent of the outer table. They do not have to refer to the same table, so this claim is incorrect.
In summary, the correct understanding is that the subquery used in the SELECT clause must return a single value, which makes option 2 the accurate choice. The other options describe conditions that are not mandatory or are explicitly incorrect.

Question at position 14 What is the requirement for using a subquery in the SELECT clause? the subquery must use an aggregate function. the subquery must return a single value. the subquery must return at least one value. the subquery must refer to the same table as the main query.单项选择题

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