Question

我有2个表：course和section，如sqlfiddle

所示

我需要为每个课程的部分选择'starttime' (or null)。有些课程没有“部分”条目。

查询：

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

返回有许多重复的条目，就像在连接表中一样。

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

当我期待：

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

我尝试了DISTINCT和GROUP BY的不同组合，但没有成功。

Answer 1

尝试：

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

Answer 2

您可以使用left join，然后使用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;

<强> SEE DEMO

Mysql在简单连接

2 个答案: