... 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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最里边的子查询找出所有需要更新的行；用户可能已经给出了一些条件来限制这些行。
The inner-most sub-select identifies all rows that need to be updated; the user might have given some conditions to restrict the rows.
子查询一般用来返回一个单独的行作为原子值，然而还可以用来比较多行的值，利用IN关键字。
Subqueries are generally used to return a single row as an atomic value although they may be used to compare values against multiple rows with the IN keyword.
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