加入两个表[重复]

Question

我有两个表：“Project”和“Project Monthly”。

| PROJECT_ID |  TITLE |    | ID | PROJECT_ID |  MONTH | EXPENSE |
-----------------------    --------------------------------------
|          1 | title1 |    |  1 |          1 |      1 |     100 |
|          2 | title2 |    |  2 |          1 |      2 |    2000 |
|          3 | title3 |    |  3 |          1 |      3 |     900 |
                           |  4 |          1 |      4 |     900 | 
                           |  5 |          2 |      1 |     200 |
                           |  6 |          2 |      2 |     200 |
                           |  7 |          3 |      1 |     500 | 

我想要一张这样的桌子：

| PROJECT_ID |  TITLE | MONTH_1 | MONTH_2 | MONTH_3 | MONTH_4 |
---------------------------------------------------------------
|          1 | title1 |     100 |    2000 |     900 |     900 |
|          2 | title2 |     200 |     200 |     NULL|     NULL|
|          3 | title3 |     500 |    NULL |     NULL|     NULL|

我可以只使用 JOIN 而不使用子查询来实现它吗？

谢谢！

Answer 1

您可以通过使用带有 max 的案例来获得所需的结果集，如果您有超过 4 个月的时间，下面的查询只会给出 4 个月的结果，那么您必须编写所有月份的案例，并选择一个最大值一个月的值中有更大的值，我使用了内部联接，因此如果任何项目没有每月数据并且您仍然希望它返回结果将内部联接更改为左联接，则 ProjectMonthly 中存在的项目只有这些项目将被返回

SELECT p.PROJECT_ID,p.TITLE,
MAX(CASE WHEN MONTH=1 THEN EXPENSE END) AS Month_1,
MAX(CASE WHEN MONTH=2 THEN EXPENSE END) AS Month_2,
MAX(CASE WHEN MONTH=3 THEN EXPENSE END) AS Month_3,
MAX(CASE WHEN MONTH=4 THEN EXPENSE END) AS Month_4
FROM Project p 
INNER JOIN ProjectMonthly pm 
ON p.PROJECT_ID=pm.PROJECT_ID
GROUP BY p.PROJECT_ID

Fiddle Demo

Answer 2

这样试试

select p.PROJECT_ID,p.TITLE,
case when month=1 then EXPENSE end as Month_1,
case when month=2 then EXPENSE end as Month_2,
case when month=3 then EXPENSE end as Month_3,
case when month=4 then EXPENSE end as Month_4
from Project p inner join ProjectMonthly' pm on p.PROJECT_ID=pm.PROJECT_ID

Answer 3

将行变成列称为透视。一种方法是group by 每个项目。该组将成为单行。在组内，您每月定义一列。该列汇总了特定月份的费用：

select  p.PROJECT_ID
,       p.TITLE
,       sum(case when m.month = 1 then m.expense end) as MONTH_1
,       sum(case when m.month = 2 then m.expense end) as MONTH_2
...
from    project p
join    monthly m 
on      p.PROJECT_ID = m.PROJECT_ID
group by 
        p.PROJECT_ID
,       p.TITLE