[英]MySQL: Select MAX() from sub-query with COUNT()
在将此标记为重复之前,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
得到一个结果:
| 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 |
这应该是这样的:
| MAX(BOOK_COUNT) | AUTHOR_ID | FN | LN |
------------------------------------------------
| 2 | 12 | name1 | lname1 |
您认为查询中有什么问题?
您可以简单地使用LIMIT
代替MAX()
。 也可以使用JOIN
。
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 |
编辑:
如果你想使用MAX()
,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 )
EDIT2:
您可以在内部查询中使用LIMIT
,而不是在外部查询中使用LIMIT
:
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
实际上,MySQL缺乏对SQL标准的支持,因为它允许使用带有GROUP BY子句的聚合函数并在结果中返回随机数据。 您应该以这种方式避免使用聚合。
编辑:我的意思是,例如在MySQL中你可以像这样执行查询:
SELECT
MAX(a), b, c
FROM
table
GROUP BY
b;
它返回c列中的随机数据,这非常错误。
我想知道这个查询正在运行,你使用聚合函数而不使用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)
如果有效,请告诉我。
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