MySQL：使用COUNT（）從子查詢中選擇MAX（）

Question

在將此標記為重復之前，請查看此SQLFiddle 。

我有這個架構：

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');

子查詢：

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

得到一個結果：

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

現在，這里棘手的部分是這個查詢的結果：

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

這是：

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

這應該是這樣的：

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

您認為查詢中有什么問題？

Answer 1

您可以簡單地使用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

輸出：

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

看到這個SQLFiddle

---

編輯：

如果你想使用MAX() ，你必須使用這樣的子查詢：

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 )

看到這個SQLFiddle

---

EDIT2：

您可以在內部查詢中使用LIMIT ，而不是在外部查詢中使用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

看到這個SQLFiddle

Answer 2

實際上，MySQL缺乏對SQL標准的支持，因為它允許使用帶有GROUP BY子句的聚合函數並在結果中返回隨機數據。 您應該以這種方式避免使用聚合。

編輯：我的意思是，例如在MySQL中你可以像這樣執行查詢：

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

它返回c列中的隨機數據，這非常錯誤。

Answer 3

我想知道這個查詢正在運行，你使用聚合函數而不使用group by。 當您需要識別具有最大角落數的用戶時請嘗試

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，請告訴我。