如何在單個MySQL語句中使用CASE加JOIN加GROUP BY

Question

SELECT B. * , SC.cate_name, (
CASE WHEN special_offer_type =  'Fixed Value'
THEN B.price - special_offer
WHEN special_offer_type =  'Discount %'
THEN B.price * ( 1 - special_offer / 100.0 ) 
ELSE B.price
END
) AS final_price
FROM book B
JOIN setting_category SC ON B.cate_id = SC.cate_id

在上面的編碼CASE和JOIN ，我在ideal_result_table （THX Gordon Linoff）處添加了兩個字段cate_name和final_price

現在，我想在ideal_result_table處再計算2個稱為avg_rate和count_comment的ideal_result_table ，我該怎么做？

用戶AAA和BBB向圖書001添加了評分，因此圖書001的avg_rate為（4 + 5）/ 2 = 4.5 = 5

用戶CCC向書002添加了評分，因此書002的avg_rate為2

用戶XXX和YYY向書001添加了注釋，因此書001的count_comment = 2

book
-----------------------------------------------------------                     
isbn   cate_id   price   special_offer   special_offer_type
001    1         125     5               Fixed Value
002    1         90      30              Discount %
003    2         150     50              Fixed Value

setting_category
--------------------                            
cate_id   cate_name
1         Fiction
2         Dictionary

book_rating                             
------------------------------------------
user   dateadd      timeadd    isbn   rate
AAA    2014/03/20   15:00:00   001     4
BBB    2014/03/21   15:00:00   001     5
CCC    2014/03/22   15:00:00   002     2

book_comment
----------------------------------------------
user    dateadd      timeadd    isbn   comment
XXX     2014/03/20   16:00:00   001    good
YYY     2014/03/21   16:00:00   001    great

ideal_result_table
-----------------------------------------------------------------------------------------------------------------
isbn   cate_id   price   special_offer   special_offer_type   cate_name    final_price   avg_rate   count_comment
001    1         125     5               Fixed Value          Fiction      120           5          2
002    1         90      30              Discount %           Fiction      63            2          0
003    2         150     50              Fixed Value          Dictionary   100           0          0

Answer 1

嘗試這個 ：

SELECT B.*,SC.cate_name,( CASE WHEN special_offer_type ='Fixed Value' THEN B.price - special_offer WHEN special_offer_type = 'Discount %' THEN B.price * ( 1 - special_offer / 100.0 ) ELSE B.price END ) AS final_price, IFNULL(xx.avg_rate,'0'), IFNULL(yy.count_comment,'0') FROM book B JOIN setting_category SC ON B.cate_id = SC.cate_id LEFT JOIN (select a.isbn,sum(a.rate)/count(a.rate) as avg_rate from book_rating a group by a.isbn) as xx on b.isbn = xx.isbn LEFT JOIN (select c.isbn,count(*) as count_comment from book_comment c group by c.isbn) as yy on b.isbn = yy.isbn

II

編輯..