相关疑难解决方法(0)

为什么你需要把你自己创建的(例如列select 1 as "number"后)HAVING,而不是WHERE在MySQL？

是否有任何缺点而不是做WHERE 1(编写整个定义而不是列名)？

我有以下两个表:

1. Lecturers (LectID, Fname, Lname, degree).
2. Lecturers_Specialization (LectID, Expertise).

我想找到专业化程度最高的讲师.当我尝试这个时,它不起作用:

SELECT
  L.LectID, 
  Fname, 
  Lname 
FROM Lecturers L, 
     Lecturers_Specialization S
WHERE L.LectID = S.LectID
AND COUNT(S.Expertise) >= ALL (SELECT
  COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);

但是当我尝试这个时,它有效:

SELECT
  L.LectID,
  Fname,
  Lname 
FROM Lecturers L,
     Lecturers_Specialization S
WHERE L.LectID = S.LectID
GROUP BY L.LectID,
         Fname,
         Lname 
HAVING COUNT(S.Expertise) >= ALL (SELECT
  COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID); 

是什么原因？谢谢.