获取列的总和而不删除一些记录

Question

以下是表格中的一些数据

slm

id	grade_level	period
1	Grade 1	1st Grading Period
2	Grade 2	2nd Grading Period

slm_activities

id	slmid	activity	count
1	1	77	5
2	2	72	6

时期

id	period
1	1st Grading Period
2	2nd Grading Period
3	3rd Grading Period
4	4th Grading Period

活动

id	activity
72	lesson
75	page
77	quiz

我需要为 mysql 获取这种输出，它将显示所有类型的活动和所有时间段，然后将它们的计数值相加，如果没有遇到其他活动，activitycount 将显示 0时期

activitycount	activity	period
5	quiz	1st Grading Period
0	quiz	2nd Grading Period
0	quiz	3rd Grading Period
0	quiz	4th Grading Period
0	lesson	1st Grading Period
6	lesson	2nd Grading Period
0	lesson	3rd Grading Period
0	page	1st Grading Period
0	page	2nd Grading Period
0	page	3rd Grading Period
0	page	4th Grading Period

这是我当前的 mysql 查询

SELECT SUM(count) AS activitycount, activities.activity, periods.period
FROM slm_activities 
LEFT JOIN activities ON activities.id = slm_activities.activity 
LEFT JOIN slm ON slm_activities.slmid = slm.id 
LEFT JOIN periods ON periods.period = slm.period
GROUP BY activities.activity, periods.period

Answer 1

由于您需要显示每个活动的所有时间段，因此您需要为此使用 CROSS JOIN。此外 table slm 应该存储 periods table 的 id 而不是 period 字段。

SELECT SUM(IFNULL(count, 0)) AS activitycount, activities.activity, periods.period
FROM activities
CROSS JOIN periods
LEFT JOIN slm ON periods.period = slm.period
LEFT JOIN slm_activities ON activities.id = slm_activities.activity
 AND slm.id = slm_activities.slmid
GROUP BY activities.activity, periods.period
ORDER BY activities.activity, periods.period;

结果

activitycount	activity	period
0	lesson	1st Grading Period
6	lesson	2nd Grading Period
0	lesson	3rd Grading Period
0	lesson	4th Grading Period
0	page	1st Grading Period
0	page	2nd Grading Period
0	page	3rd Grading Period
0	page	4th Grading Period
5	quiz	1st Grading Period
0	quiz	2nd Grading Period
0	quiz	3rd Grading Period
0	quiz	4th Grading Period

获取列的总和而不删除一些记录

getting sum of column and not delete some records

mysql

sql

join

参见 this fiddle。