Question

我有三个表格排列如下

Person               Activity               Person_Activity
-------------------  --------------------   -----------------------------
ID | Name | Dept     ID | Activity | Type   PersonId | ActivityId | Value
-------------------  --------------------   -----------------------------
1    David  2        1   act1        1      2         1             5
2    Jim    1        2   act2        2      2         2             1 
3    Steve  1        3   act3        1      3         3             3
                                            1         1             1

我想为活动类型1的部门1中的所有人选择值。当Person_Activity中有值时，这很容易，但是当我没有时，我仍然需要列出该人员和值为0的活动.EG。

PersonName | ActivityName | Value
Jim          act1           5
Jim          act3           0
Steve        act1           0
Steve        act3           3

我尝试从Person_activity表中选择并使用正确的联接来加入人和活动但是这对我不起作用。任何人都可以指出正确的方向。

Answer 1

SELECT p.Name, a.Name, CASE WHEN (pa.Value IS NOT NULL) THEN pa.Value ELSE 0 END
FROM Person p
CROSS JOIN Activity a 
LEFT JOIN Person_Activity pa ON p.ID = pa.PersonId AND a.ID = pa.ActivityId
WHERE p.Dept = 1 and a.Dept = 1
ORDER BY 1, 2

并尝试小提琴试试：http://sqlfiddle.com/#!2/03d8b/2

Answer 2

为什么要使用CROSSJOIN？为什么在p.Dept和a.Dept过滤时它们是相同的？

这更简单

http://sqlfiddle.com/#!2/9c6537/3

SELECT p.Name, a.Activity, CASE WHEN (pa.Value is null) THEN 0 else pa.Value END
from Person p
left join Activity a on a.Type = p.Dept
left join Person_Activity pa on pa.PersonId = p.Id and pa.ActivityId = a.Id
where p.Dept = 1 and a.Type = 1
order by p.Name, a.Activity

如果a.Type字段与p.Dept不同，则这是解决方案：

http://sqlfiddle.com/#!2/9c6537/5

SELECT p.Name, a.Activity, CASE WHEN (pa.Value is null) THEN 0 else pa.Value END
from Person p
left join Activity a on a.Type = 1
left join Person_Activity pa on pa.PersonId = p.Id and pa.ActivityId = a.Id
where p.Dept = 1 
order by p.Name, a.Activity

MYSQL - 加入三个表

2 个答案: