... FROM (SELECT SUM(column1) AS sum_column1 FROM t1 GROUP BY column1) AS t1; 行的子查询（Row Subquery）：
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Single-row subquery 单行子查询
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”。
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