Mysql的返回重複簡單的條目加入

Question

我有2個表：course和section如圖sqlfiddle

我需要選擇'starttime' (or null)每門課程的部分。 有些課程沒有「部分」條目。

查詢：

select section.id, section.course, section.section, IFNULL(schedule.starttime, "null") 
from section 
join schedule on schedule.courseid = section.course 
where course = 3 and section.section > 0 

回報有許多重複的條目中有連接表。

id  course section IFNULL(schedule.starttime, "null") 
7  3  1  1440021600 
7  3  1  1440021620 
7  3  1  1440021640 
8  3  2  1440021600 
8  3  2  1440021620 
8  3  2  1440021640 
9  3  3  1440021600 
9  3  3  1440021620 
9  3  3  1440021640 

當我想到：

id  course section IFNULL(schedule.starttime, "null") 
7  3  1  1440021600 
8  3  2  1440021620 
9  3  3  1440021640 
10  3  4  null 
11  3  5  null 

我試過的DISTINCT和GROUP BY不同的組合沒有成功。

Answer 1

可以使用left join然後使用schedule.sectionid和section.id

select section.id, section.course, section.section, IFNULL(schedule.starttime, "") 
from section 
LEFT JOIN schedule ON schedule.sectionid = section.id 
where course = 3 and section.section > 0; 

SEE DEMO

Answer 2

嘗試：

SELECT section.id, section.course, section.section, IFNULL(schedule.starttime, "null") 
FROM section 
LEFT JOIN schedule ON schedule.courseid = section.course AND schedule.id = section.section 
WHERE course = 3 AND section.section > 0