mysql - MAX与Count相减

Question

我想知道一位作者的最大图书数量与每位作者的图书数量之差。

SELECT  count(book_name) contar, (MAX(C.qty) - count(book_name)) final
FROM (select B.author_id, count(book_name) qty 
     FROM author A ,BOOK B 
     WHERE A.author_id = B.author_id 
     GROUP BY b.author_id) C , 
author A, 
BOOK B 
WHERE A.author_id = B.author_id 
    AND B.author_id=C.Author_id
    AND date_release BETWEEN NOW() - INTERVAL 20 YEAR AND NOW()
GROUP BY c.author_id
HAVING contar > 0

如您所见，有两个表 BOOK，其中包含 book_id、author_id、book_name、date_release 和表 AUTHOR author_id、author_name、author_lastname、date_birth .

我得到了这个结果

但是我需要类似的东西

--------+-------+--+
| contar | final |  |
+--------+-------+--+
|      1 |    -4 |  |
|      2 |    -3 |  |
|      5 |     0 |  |
|      1 |    -4 |  |
+--------+-------+--+ 

Answer 1

问题是 MAX(C.qty) 是按作者计算的，因为查询有 GROUP BY c.author_id。您只需为整个子查询计算一次总数。

在获取每个作者的书籍数量时，也不需要加入 AUTHOR，因为作者 ID 在 BOOK 中。

SELECT  count(book_name) contar, (max_qty - count(book_name)) final
FROM (SELECT MAX(qty) AS max_qty
      FROM (select count(book_name) qty 
            FROM BOOK B 
            GROUP BY b.author_id) t
     ) C 
CROSS JOIN author A
JOIN book B ON A.author_id = B.author_id 
WHERE date_release BETWEEN NOW() - INTERVAL 20 YEAR AND NOW()
GROUP BY A.author_id

并且不需要 HAVING contar > 0，因为没有书籍的作者将不会包含在 JOIN 中。