Question

我想计算出租借的书籍数量最多的主题区域。我有以下查询：

SELECT SubjectArea
FROM (

  SELECT SubjectArea, MAX( Copies_On_Loan ) AS Max
  FROM (

    SELECT
      Section.SubjectArea AS SubjectArea, 
      SUM( LoanBook.Copies_On_Loan ) AS Copies_On_Loan
    FROM Section
      NATURAL JOIN Items
      NATURAL JOIN LoanBook
    GROUP BY Section.SubjectArea
  ) AS Table1
) AS Table2

最里面的查询：

    SELECT 
      Section.SubjectArea AS SubjectArea, 
      SUM( LoanBook.Copies_On_Loan ) AS Copies_On_Loan
    FROM Section
      NATURAL JOIN Items
      NATURAL JOIN LoanBook
    GROUP BY Section.SubjectArea

返回下表：

SubjectArea Copies_On_Loan
Biology         0
DBMS            3

但是，整个查询将结果显示为Biology（而不是DBMS）。请说明为什么会发生这种情况

Answer 1

好吧，我想我明白你想要什么 - 你正试图获得拥有Max（Copies_On_Loan）的主题。如果是这样，这应该工作（如果MySQL支持CTE会更漂亮）：

SELECT SubjectArea
FROM (
  SELECT MAX( Copies_On_Loan ) AS Max_Copies_On_Loan 
  FROM (
    SELECT
      Section.SubjectArea AS SubjectArea, 
      SUM( LoanBook.Copies_On_Loan ) AS Copies_On_Loan
    FROM Section
      NATURAL JOIN Items
      NATURAL JOIN LoanBook
    GROUP BY Section.SubjectArea
  ) AS Table1
) AS Table2 JOIN 
(
   SELECT
      Section.SubjectArea AS SubjectArea, 
      SUM( LoanBook.Copies_On_Loan ) AS Copies_On_Loan
    FROM Section
      NATURAL JOIN Items
      NATURAL JOIN LoanBook
    GROUP BY Section.SubjectArea
) Table3 ON Table2.Max_Copies_On_Loan  = Table3.Copies_On_Loan

以下是更新后的SQL Fiddle。

祝你好运。

Answer 2

在我的头顶，看起来你需要类似的东西：

SELECT SubjectArea
FROM Section NATURAL JOIN Items NATURAL JOIN LoanBook
GROUP BY SubjectArea
HAVING SUM(Copies_On_Loan) = (
    SELECT MAX(M) (
        SELECT SUM(Copies_On_Loan) M
        FROM Section NATURAL JOIN Items NATURAL JOIN LoanBook
        GROUP BY SubjectArea
    ) Q1
) Q2

计划英语：

汇总每个主题的贷款（SUM(Copies_On_Loan) M）并找到最大金额（MAX(M)）。
然后只选择总和与最大值（HAVING）匹配的结果。

P.S。：正如其他人所建议的那样，使用显式列名（代替NATURAL JOIN）将使您的代码更少＆＃34;脆弱＆＃34;。

P.P.S。：您应该为这些问题提供数据库结构，甚至更好地提供一个有效的sqlfiddle.com示例，以便我们立即测试答案。

检索包含字段最大值的整个行

2 个答案: