不能在 SQL*Plus 中使用 Max 和 Count

Question

这是我的 sql 语句，我收到此错误。但是当我只使用 Max 单并且不显示其他结果时它可以工作。有人可以帮我吗

SELECT cat.CategoryName,sb.SubCategoryName,MAX((COUNT(bs.BookID))) 
FROM
  Category cat,SubCategory sb, Book_Subcategory bs 
WHERE cat.CategoryID = sb.CategoryID AND sb.SubCategoryID = bs.SubCategoryID 
GROUP BY cat.CategoryName, sb.SubCategoryName, bs.BookID;

ERROR at line 1:
ORA-00937: not a single-group group function

有人可以帮我吗？

Answer 1

SQL 不允许直接聚合聚合。

但是，如果在 FROM 子句中的子查询中编写内部聚合（或使用 WITH 子句和公共表表达式，CTE），则可以实现结果：

SELECT gc1.CategoryName, gc1.SubCategoryName, gc1.BookCount
  FROM (SELECT cat.CategoryName, sb.SubCategoryName,
               COUNT(bs.BookID) AS BookCount
          FROM Category AS cat
          JOIN SubCategory AS sb ON cat.CategoryID = sb.CategoryID
          JOIN Book_Subcategory AS bs ON sb.SubCategoryID = bs.SubCategoryID
         GROUP BY cat.CategoryName, sb.SubCategoryName
       ) AS gc1
 WHERE gc1.BookCount = (SELECT MAX(gc2.BookCount)
                          FROM (SELECT cat.CategoryName, sb.SubCategoryName,
                                       COUNT(bs.BookID) AS BookCount
                                  FROM Category AS cat
                                  JOIN SubCategory AS sb
                                       ON cat.CategoryID = sb.CategoryID
                                  JOIN Book_Subcategory AS bs
                                       ON sb.SubCategoryID = bs.SubCategoryID
                                 GROUP BY cat.CategoryName, sb.SubCategoryName
                                ) AS gc2
                        )

这很复杂，因为它不使用 CTE，并且有一个必须写出两次的公用表表达式。

使用 CTE 形式（可能有语法错误）：

WITH gc1 AS (SELECT cat.CategoryName, sb.SubCategoryName,
                    COUNT(bs.BookID) AS BookCount
               FROM Category AS cat
               JOIN SubCategory AS sb
                    ON cat.CategoryID = sb.CategoryID
               JOIN Book_Subcategory AS bs
                    ON sb.SubCategoryID = bs.SubCategoryID
              GROUP BY cat.CategoryName, sb.SubCategoryName
            )
SELECT gc1.CategoryName, gc1.SubCategoryName, gc1.BookCount
  FROM gc1
 WHERE gc1.BookCount = SELECT MAX(gc1.BookCount) FROM gc1);

更整洁！

如果您的 DBMS 可以轻松创建临时表，则您可以使用临时表模拟 CTE。例如，IBM Informix Dynamic Server 可以使用：

SELECT cat.CategoryName, sb.SubCategoryName,
       COUNT(bs.BookID) AS BookCount
  FROM Category AS cat
  JOIN SubCategory AS sb ON cat.CategoryID = sb.CategoryID
  JOIN Book_Subcategory AS bs ON sb.SubCategoryID = bs.SubCategoryID
 GROUP BY cat.CategoryName, sb.SubCategoryName
  INTO TEMP gc1;

SELECT gc1.CategoryName, gc1.SubCategoryName, gc1.BookCount
  FROM gc1
 WHERE gc1.BookCount = (SELECT MAX(gc1.BookCount) FROM gc1);

DROP TABLE gc1;  -- Optional: table will be deleted at end of session anyway

---

鉴于以下表格和数据，在 MacOS X 10.6.4 上针对 IBM Informix Dynamic Server 11.50.FC6 运行时，主查询（从该答案复制和粘贴）给出了我预期的结果，即：

Non-Fiction    SQL Theory    4
Fiction        War           4

这并不能证明它在针对 Oracle 运行时“必须工作” - 我没有 Oracle，也无法演示任何一种方式。它确实表明至少有一个 SQL DBMS 可以毫无问题地处理查询。 （由于 IDS 不支持 WITH 子句和 CTE，我无法证明该公式是否有效。）

架构

CREATE TABLE Category
(
    CategoryID INTEGER NOT NULL PRIMARY KEY,
    CategoryName VARCHAR(20) NOT NULL
);
CREATE TABLE SubCategory
(
    CategoryID INTEGER NOT NULL REFERENCES Category,
    SubCategoryID INTEGER NOT NULL PRIMARY KEY,
    SubCategoryName VARCHAR(20) NOT NULL
 );
CREATE TABLE Book_SubCategory
(
    SubCategoryID INTEGER NOT NULL REFERENCES SubCategory,
    BookID INTEGER NOT NULL PRIMARY KEY
);

数据

INSERT INTO Category VALUES(1, 'Fiction');
INSERT INTO Category VALUES(2, 'Non-Fiction');

INSERT INTO SubCategory VALUES(2, 1, 'SQL Theory');
INSERT INTO SubCategory VALUES(2, 2, 'Mathematics');
INSERT INTO SubCategory VALUES(1, 3, 'Romance');
INSERT INTO SubCategory VALUES(1, 4, 'War');

INSERT INTO Book_SubCategory VALUES(1, 10);
INSERT INTO Book_SubCategory VALUES(2, 11);
INSERT INTO Book_SubCategory VALUES(3, 12);
INSERT INTO Book_SubCategory VALUES(3, 13);
INSERT INTO Book_SubCategory VALUES(4, 14);
INSERT INTO Book_SubCategory VALUES(1, 15);
INSERT INTO Book_SubCategory VALUES(1, 16);
INSERT INTO Book_SubCategory VALUES(2, 17);
INSERT INTO Book_SubCategory VALUES(1, 18);
INSERT INTO Book_SubCategory VALUES(3, 19);
INSERT INTO Book_SubCategory VALUES(4, 20);
INSERT INTO Book_SubCategory VALUES(4, 21);
INSERT INTO Book_SubCategory VALUES(4, 22);

Answer 2

我认为错误出在 GROUP BY 子句中（bs.BookID 不属于那里）：

SELECT cat.CategoryName,sb.SubCategoryName,MAX(COUNT(bs.BookID)) 
FROM Category cat,SubCategory sb, Book_Subcategory bs 
WHERE cat.CategoryID =sb.CategoryID AND sb.SubCategoryID=bs.SubCategoryID 
GROUP BY cat.CategoryName,sb.SubCategoryName;

顺便说一句，空格（和标点符号）是你的朋友。不要偷懒。