JOIN不顯示所有行

Question

我有以下查詢。

SELECT WEEK(Review.created)        AS Week, 
     CONCAT(Employee.firstname, ' ', Employee.lastname) AS Name, 
     AVG(Rating.scale)         AS Average 
FROM employees Employee 
     LEFT JOIN reviews Review 
     ON Employee.id = Review.reviewee_id 
     LEFT JOIN ratings AS Rating 
     ON Rating.id = Review.rating_id 
WHERE Employee.id IN (71, 72) 
GROUP BY WEEK(Review.created), 
      Employee.id 
ORDER BY WEEK(Review.created), 
      Employee.id 

結果是這樣的：

36, Employee1, 2.9091 
37, Employee2, 3.5000 
37, Employee1, 3.7143 
38, Employee2, 4.2000 
38, Employee1, 4.0000 
39, Employee2, 2.0000 
40, Employee2, 2.8333 
40, Employee1, 3.8571 
41, Employee1, 2.6667 
43, Employee2, 2.5000 
43, Employee1, 1.5714 
44, Employee2, 3.8333 
44, Employee1, 4.4000 
45, Employee2, 3.2500 
45, Employee1, 4.8571 
46, Employee2, 2.1667 
46, Employee1, 2.2000 
48, Employee2, 2.6667 
49, Employee2, 1.4000 
49, Employee1, 3.5000 
50, Employee2, 2.0000 
50, Employee1, 1.5000 
51, Employee2, 2.7143 
51, Employee1, 2.7500 

我需要的是返回的員工，即使他們沒有爲這一週的評級。

所以前兩行會返回

36, Employee1, 2.9091 
36, Employee2, NULL 

任何幫助，在此將不勝感激。

Answer 1

可能是因爲查詢中的分組結果被省略。嘗試刪除分組，並看看會發生什麼

來源

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html

Answer 2

變化by子句您的團隊

GROUP BY Employee.id, WEEK(Review.created) 

因爲目前的情況是，如果集團首先在回顧一週，任何對於那周沒有評論的員工將首先按NULL（week(null) -> null）進行分組。