双重不存在子句是什么意思? [英] What does a double not exists clause mean?

问题描述

SELECT c.name
FROM   Customer c
WHERE  NOT EXISTS(SELECT w.WID
                  FROM   Woker w
                  WHERE  NOT EXISTS(SELECT la
                                    FROM   look_after la
                                    WHERE  la.CID = c.CID
                                           AND la.WID = w.WID)); 

我不知道代码的含义...谁能大致告诉我代码的作用? C是一个客户，它将从一个工人那里得到照顾.

I dont know what the code means... Could anyone tell me broadly what the code do? C is a Customer, who will looked after from a Worker.

推荐答案

该查询选择所有工作人员照顾的客户.

The query selects customers that are looked after by all workers.

双重not exists是一种实现 查看全文