Note: To detect a cycle, only the results returned by the iterative SELECT statement are checked.

注意：要检测循环，在检测循环时，系统只会检查迭代select语句返回的结果。

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The result of the current iterative select statement is merged with the result of the initial select statement and all results returned by the previous iterations using a UNION ALL clause.

使用 UNION ALL 子句，可将当前的迭代 SELECT 语句的结果与初始 SELECT 语句的结果以及之前的迭代返回的所有结果合并在一起。

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