MySQL：查找最後x個記錄的每個條目的平均值

Question

我試圖弄清楚如何獲取每個銷售人員在過去的100條評級中的平均評級（如果他們目前就業），並且他們的平均評級少於3（滿分5）。

我有（未在查詢所需留出的信息）下表：

users 
id name  employed 
----------------------- 
1  John  1 
2  Sue  1 
3  Bob  0 
... 

sales 
id  users_id 
------------------ 
100 3 
101 2 
102 3 
103 1 
... 

ratings 
sales_id rating 
----------------- 
100  4 
101  5 
102  5 
103  2 
... 

當前查詢我有搜索一切，曾經返回平均值爲所有的訂單，但我希望它只抓取最近的100個評級（如果銷售人員沒有銷售那麼多項目，則評分更低），但不包括任何不再被僱用或對其最近100個訂單的評級大於3的人員。這是當前查詢：

SELECT u.name, avg(r.rating) as avg_rating, count(r.rating) 
FROM users AS u 
JOIN sales AS s ON s.users_id = u.id 
JOIN ratings AS r ON r.sales_id = s.id 
WHERE u.employed = 1 
GROUP BY u.id 
HAVING avg_rating <= 3; 

任何幫助將是偉大的！謝謝！ ：D

Answer 1

您可以使用我的sql變量來跟蹤評級的數量，以便您只能獲得最近的100個評級，通過sales_id進行排序，以便獲得最近的評級。

SQL FIDDLE DEMO

SELECT T.name, avg(T.rating) as avg_rating, count(T.rating) 
FROM 
(
    SELECT u.name, r.rating, @num := if (@name = name, @num+1, 1) as rn, 
     @name:= name as var_name 
    FROM users AS u 
    JOIN sales AS s ON s.users_id = u.id 
    JOIN ratings AS r ON r.sales_id = s.id 
    AND u.employed = 1 
    JOIN (select @name :='' , @num :=1) var 
    order by sales_id desc 
)T 
where T.rn <=100 
GROUP BY T.name 
HAVING avg_rating <= 3