SQL - HAVING 与 WHERE

问题描述

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.当我尝试此操作时，它不起作用:

I want to find the lecturer with the most Specialization.When I try this, it is not working:

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，它起作用了:

But when I try this, it works:

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.

推荐答案

WHERE 子句在单个行上引入了条件；HAVING 子句在 aggregations 上引入了条件，即选择的结果，其中单个结果，例如 count、average、min、max 或 sum，已经从 多行.您的查询需要第二种条件(即聚合条件)，因此 HAVING 可以正常工作.

WHERE clause introduces a condition on individual rows; HAVING clause introduces a condition on aggregations, i.e. results of selection where a single result, such as count, average, min, max, or sum, has been produced from multiple rows. Your query calls for a second kind of condition (i.e. a condition on an aggregation) hence HAVING works correctly.

根据经验，在GROUP BY 之前使用WHERE，在GROUP BY 之后使用HAVING.这是一个相当原始的规则，但在 90% 以上的情况下都是有用的.

As a rule of thumb, use WHERE before GROUP BY and HAVING after GROUP BY. It is a rather primitive rule, but it is useful in more than 90% of the cases.

在此过程中，您可能希望使用 ANSI 版本的连接重新编写查询:

While you're at it, you may want to re-write your query using ANSI version of the join:

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

这将消除用作theta 连接条件的WHERE.

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