接下来，读线程需要确保(这是第二个检查)它等待条件变量的时间不超过指定的超时时间。
Next, the reader thread needs to ensure (and this is the second check you perform) that it does not wait on the condition variable any more than the specified timeout period.
由于我们没有指定任何连接条件，所以第一个结果集中的每一行将会连接第二个中的每一行，从而产生57*21行。 。
Since eI have not specified any join condition, each row from the first result set will join with eachrow from the second one, producing 57*21 rows.
相反，更新查询可查找与指定条件相匹配的记录，然后用称为更新条件的第二个条件替换这些记录。
In contrast, an update query finds records that match the criteria that you specify, and then it replaces those records with a second criteria called an update criteria.
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