Question

我正在尝试创建一个查询，向我显示2个表之间的合并数据

当前的表格如下：

person:ID,
Name,
Age


Form
ID,
Person_ID,
question,
answer


Example data would be
1 |Brandon| 55
2 |John |88
3 |Bob| 100


Form:
1 |1 |do you have diabetes |Y
2 |1 |do you take medications| Y
3 |1| how many times a week |7
4 |2|do you have diabetes |N

我尝试过以下操作：

SELECT 
p.Name
er1.answer,
er2.answer,
er3.answer
from person p
LEFT JOIN form er1 ON p.ID = er1.Person_ID
LEFT JOIN form er2 ON p.ID = er2.Person_ID 
LEFT JOIN form er3 ON p.ID = er3.Person_ID
where er1.question='do you have diabetes'
and er2.question='do you take medications'
and er3.question='how many times a week'

问题是第2个人没有出现，因为他们回答“否”，数据库没有回答其他2个问题。有没有一种方法可以让查询搜索并提取数据，如果不存在，则在那里只有一个空值。

Answer 1

移动ON子句中的条件并删除WHERE子句：

SELECT 
  p.Name,
  er1.answer,
  er2.answer,
  er3.answer
from person p
LEFT JOIN form er1 ON p.ID = er1.Person_ID AND er1.question='do you have diabetes'
LEFT JOIN form er2 ON p.ID = er2.Person_ID AND er2.question='do you take medications'
LEFT JOIN form er3 ON p.ID = er3.Person_ID AND er3.question='how many times a week'

最好给列起别名：

SELECT 
  p.Name,
  er1.answer as `do you have diabetes`,
  er2.answer as `do you take medications`,
  er3.answer as `how many times a week`
from person p
LEFT JOIN form er1 ON p.ID = er1.Person_ID AND er1.question='do you have diabetes'
LEFT JOIN form er2 ON p.ID = er2.Person_ID AND er2.question='do you take medications'
LEFT JOIN form er3 ON p.ID = er3.Person_ID AND er3.question='how many times a week'

请参见demo。
结果：

| Name    | do you have diabetes | do you take medications | how many times a week |
| ------- | -------------------- | ----------------------- | --------------------- |
| Brandon | Y                    | Y                       | 7                     |
| John    | N                    |                         |                       |
| Bob     |                      |                         |                       |

现在考虑通过条件聚合获得所需结果的另一种方法：

select 
  p.Name,
  max(case question when 'do you have diabetes' then answer end) as `do you have diabetes`,
  max(case question when 'do you take medications' then answer end) as `do you take medications`,
  max(case question when 'do you have diabetes' then answer end) as `how many times a week`
from person p LEFT JOIN form f ON p.ID = f.Person_ID 
group by p.ID, p.Name

请参见demo。

包括不存在数据的左联接中的行

1 个答案: