如何在MySQL中添加平均值w/sum（case when ...）？

Question

我正在處理一個幾乎在我想要的地方的SQL查詢。我只想爲每個「attributes_share_lemon」和「attribites_not_shared_lemon」的平均提交添加兩列。

這裏是SQL小提琴，我是工作在這裏：

http://sqlfiddle.com/#!9/6ea5d8/8

這裏是創建和插入：

CREATE TABLE `fruits` (
    `id` int(11) unsigned NOT NULL AUTO_INCREMENT, 
    `fruit` varchar(11) DEFAULT NULL, 
    `fruit_attribute` varchar(11) DEFAULT '', 
    `submissions` int(11) DEFAULT NULL, 
    PRIMARY KEY (`id`) 
); 

INSERT INTO `fruits` (`id`, `fruit`, `fruit_attribute`, `submissions`) 
VALUES 
    (1,'Orange','tough peel',59), 
    (2,'Lemon','tough peel',70), 
    (3,'Orange','citrus',100), 
    (4,'Orange','juice',90), 
    (5,'Lemon','juice',75), 
    (6,'Lemon','tart',35), 
    (7,'Lemon','citurs',65), 
    (8,'Orange','breakfast',110), 
    (9,'Lemon','lemonaid',120), 
    (10,'Orange','florida',50); 

我的查詢：

SELECT ft1.fruit, ft1.submissions, 
    SUM(CASE WHEN ft2.fruit_attribute IS NOT NULL THEN 1 ELSE 0 END) As attributes_shared_lemon, 
    SUM(CASE WHEN ft2.fruit_attribute IS NULL THEN 1 ELSE 0 END) As attributes_not_shared_lemon 
FROM fruits ft1 LEFT JOIN 
(
    SELECT ft.fruit, ft.fruit_attribute, ft.submissions 
    FROM fruits ft 
    WHERE ft.fruit = 'Lemon' 
) ft2 
ON ft1.fruit_attribute = ft2.fruit_attribute 
GROUP BY ft1.fruit; 

CURRENT OUTPUT

| fruit | submissions | attributes_shared_lemon | attributes_not_shared_lemon | 
|--------|-------------|-------------------------|-----------------------------| 
| Lemon |   70 |      5 |       0 | 
| Orange |   59 |      2 |       3 | 

Answer 1

您的查詢看起來沒問題。但是，你並不需要的子查詢，所以一個簡單的版本是：

SELECT ft.fruit, 
     COUNT(ftl.fruit_attribute) As attributes_shared_lemon, 
     SUM(ftl.fruit_attribute IS NULL) As attributes_not_shared_lemon 
FROM fruits ft LEFT JOIN 
    fruits ftl 
    ON ft.fruit_attribute = ftl.fruit_attribute and ftl.fruit = 'Lemon' 
GROUP BY ft.fruit; 

我刪除了submissions列，因爲它不是在每一行唯一的。

編輯：

如果你想submissions列這些羣體的平均水平，使用case：

SELECT ft.fruit, 
     AVG(CASE WHEN ftl.fruit_attribute IS NOT NULL THEN ft.submissions END) as avg_shared, 
     AVG(CASE WHEN ftl.fruit_attribute IS NULL THEN ft.submissions END) as avg_notshared, 
     COUNT(ftl.fruit_attribute) As attributes_shared_lemon, 
     SUM(ftl.fruit_attribute IS NULL) As attributes_not_shared_lemon 
FROM fruits ft LEFT JOIN 
    fruits ftl 
    ON ft.fruit_attribute = ftl.fruit_attribute and ftl.fruit = 'Lemon' 
GROUP BY ft.fruit; 

Answer 2

它仍然是不明確什麼是你的問題或問題。

只是我的猜測。可能您需要SUM(ft1.submissions)

和COUNT(DISTINCT ft2.fruit_attribute)以避免多次統計相同的屬性。

和COUNT(DISTINCT ft1.fruit_attribute)-COUNT(DISTINCT ft2.fruit_attribute)確保您有正確的currnt水果屬性計數器。

http://sqlfiddle.com/#!9/6ea5d8/22

SELECT ft1.fruit, SUM(ft1.submissions), 
    COUNT(DISTINCT ft2.fruit_attribute) As attributes_shared_lemon, 
    COUNT(DISTINCT ft1.fruit_attribute)-COUNT(DISTINCT ft2.fruit_attribute) As attributes_not_shared_lemon 
FROM fruits ft1 
LEFT JOIN fruits ft2 
ON ft1.fruit_attribute = ft2.fruit_attribute 
    AND ft2.fruit = 'Lemon' 
GROUP BY ft1.fruit;