4

在将其标记为重复之前，请查看此SQLFiddle。

我有这个架构：

CREATE TABLE book(book_id int,
                   book_name varchar(100),
                   author_id int,
                   editor_id varchar(100),
                   isbn varchar(100));
INSERT INTO book
VALUES
(1 , 'Book1 Title' ,  12  ,  'Editor1'   , '8000-9000' ),
(2 , 'Book2 Title' ,  98  ,  'Editor1'   , '8000-9001' ),
(1 , 'Book1 Title' ,  12  ,  'Editor1'   , '8000-9002' ),
(3 , 'Book3 Title' ,  3  ,  'Editor1'   , '8000-9003' );

CREATE TABLE author(author_id int,
                    fn varchar(100),
                    ln varchar(100));
INSERT INTO author
VALUES
(12, 'name1','lname1'),
(98,'name2','lname2'),
(3,'name3','lname3');

子查询：

  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id

有一个结果：

| AUTHOR_ID | BOOK_COUNT |
--------------------------
|         3 |          1 |
|        12 |          2 |
|        98 |          1 |

现在，这里棘手的部分是这个查询的结果：

SELECT MAX(book_count),a.* FROM
author a,(
  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id
) b 
where a.author_id = b.author_id

这是：

| MAX(BOOK_COUNT) | AUTHOR_ID |    FN |     LN |
------------------------------------------------
|               2 |         3 | name3 | lname3 |

应该是这样的：

| MAX(BOOK_COUNT) | AUTHOR_ID |    FN |     LN |
------------------------------------------------
|               2 |        12 | name1 | lname1 |

您认为查询中有什么问题？

4

3 回答 3

4

而不是MAX()你可以简单地使用LIMIT相同的。也可以JOIN改用。

SELECT book_count,a.author_id,a.fn, a.ln 
FROM author a
JOIN
(
  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id
) b 
ON a.author_id = b.author_id
ORDER BY book_count DESC LIMIT 1

输出：

| BOOK_COUNT | AUTHOR_ID |    FN |     LN |
-------------------------------------------
|          2 |        12 | name1 | lname1 |

看到这个 SQLFiddle

编辑：

如果你想使用MAX()它，你必须使用这样的子查询：

SELECT book_count,a.author_id,a.fn, a.ln 
FROM author a
JOIN
(
  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id
) b 
ON a.author_id = b.author_id
WHERE book_count = 
        (SELECT MAX(book_count)
        FROM
        (
           SELECT c.author_id,COUNT(*) book_count FROM book c 
           GROUP BY c.author_id
        ) b )

看到这个 SQLFiddle

编辑2：

除了LIMIT在外部查询中使用，您也可以在内部查询中简单地使用它：

SELECT book_count,a.author_id,a.fn, a.ln 
FROM author a
JOIN
(
  SELECT c.author_id,COUNT(*) book_count FROM book c
  GROUP BY c.author_id
  ORDER BY COUNT(*) DESC LIMIT 1
) b 
ON a.author_id = b.author_id

看到这个 SQLFiddle

于 2013-08-05T06:27:10.190 回答
1

事实上，MySQL 缺乏对 SQL 标准的支持，因为它允许使用没有 GROUP BY 子句的聚合函数，并在结果中返回随机数据。您应该避免以这种方式使用聚合。

编辑：我的意思是，例如在 MySQL 中，您可以执行如下查询：

SELECT
  MAX(a), b, c
FROM
  table
GROUP BY
  b;

它在 c 列中返回随机数据，这是非常错误的。

于 2013-08-05T06:33:08.437 回答
0

我想知道这个查询正在运行，您使用了聚合函数而不使用 group by。当您需要识别具有最大角落数的用户时请尝试

SELECT (book_count),b.author_id FROM
author a

INNER JOIN 
(
 SELECT c.author_id,COUNT(*) book_count FROM book c
 GROUP BY c.author_id) B
ON

 a.author_id = b.author_id
having book_count=MAX(book_count)

让我知道它是否有效。

于 2013-08-05T06:30:26.140 回答