MySQL：使用COUNT()从子查询中选择MAX()

在将其标记为重复之前,请先查看此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

结果：

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

现在,这里最棘手的部分是此查询的结果：

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？

解决方法:

代替MAX(),您可以简单地使用LIMIT来实现.也可以使用JOIN代替.

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 |

查看this SQLFiddle

编辑：

如果要为此使用MAX(),则必须使用如下子查询：

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 )

参见this SQLFiddle

编辑2：

除了在外部查询中使用LIMIT之外,您也可以在内部查询中使用它：

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

参见this SQLFiddle

标签：aggregate-functions,sql,mysql
来源： https://codeday.me/bug/20191030/1968953.html