250. A subquery is called a singlerow subquery when ____. A. the inner query returns a single value to the main query B. the inner query uses an aggregate function and returns one or more values C. there is only one inner query in the main query and the inner query returns one or more values D. the inner query returns one or more values and the main query returns a single value as output Answer: A single-row function指一行数据输入，返回一个值的函数。 如substr等。 而mutil-row function指多行数据输入，返回一个值的函数。 如sum、max等。 Eg: SQL> select a,a+b ,sum(b) from test group by a,a+b; Ok
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