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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)); 

我不知道代码意味着什么...有谁能告诉我代码的作用？C是一名顾客,他将照顾一名工人.