A. a single row subquery can retrieve data from only one table.
单行子查询可以只从一个表中检索数据。
A scalar subquery is an embedded query that returns a single row that contains a single column — essentially, a single value, which is known as a scalar.
标量子查询是一种嵌入式查询，返回包含单个列的单个行，本质上就是单个值，该值称为标量。
The EXISTS condition is considered "to be met" if the subquery returns at least one row.
如果子查询中返回至少有一行，EXISTS条件才被认为是“符合条件”。
The EXISTS condition is considered "to be met" if the subquery returns at least one row.
如果子查询中返回至少有一行，EXISTS条件才被认为是“符合条件”。
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