從連接表中計算分組行

Question

我正在尋找解決方案來從連接表中計算分組行。

表參與者

id |   email   |  date 
----+---------------------------+-------------- 
1 |  john@example.com  | 14/01/2015 
2 |  sally@mailing.com  | 14/01/2015 
3 |  sally@mailing.com  | 01/01/2015 
4 |  sally@mailing.com  | 20/01/2015 
5 | edward@halloworld.com | 13/02/2015 
6 | edward@halloworld.com | 15/01/2015 

表數據

pid | fieldid | data 
----+---------+-------------- 
1 | 1 | Dutch 
1 | 2 | Cheese 
2 | 1 | German 
2 | 2 | Soup 
3 | 1 | German 
3 | 2 | Bread 
4 | 1 | German 
4 | 2 | Sausage 
5 | 1 | Italian 
5 | 2 | Meat balls 
6 | 1 | Italian 
6 | 2 | Pizza 

我想看看如何往往是一個用戶（根據他們的電子郵件地址）參加。但是，下面的查詢返回doenst預期的結果：

SELECT 
    COUNT(*) AS participated, 
    p.email, 
    coalesce(MAX(case when fieldid = 1 then data end), 0) as "language" 
FROM participants p 
LEFT JOIN data d 
    ON d.pid = p.id 
GROUP BY p.email 

結果：

participated |   email   | language 
-------------+---------------------------+------------ 
     2  |  john@example.com  | Dutch 
     6  | sally@mailing.com  | German 
     4  | edward@halloworld.com | Italian 

「計數」結果不是我所期望的。它似乎被算爲雙倍。這可能是因爲在參與者表中的每個記錄的數據表中有兩條記錄。

什麼是正確的查詢使用？

Answer 1

如果你得到的數據是你有的兩倍，那麼你大概想要count(distinct d.pid)。