如何通过选择另一个字段值将一个表与两个表联接

Question

I have three tables. first table! activity with these fields: id_company , id_act , type [ blog or poll ] second table! blog with these fields: id , 'body and third table! poll with these fields: 'id , 'text , answer`

Now, I want fetch my info from activity table by join activity with blog and poll tables depend on type field.

activity table
-----------------------------
id_company|   id_act|   type
-----------------------------
1         |   1     |   blog
1         |   2     |   blog
2         |   3     |   blog
1         |   1     |   poll
2         |   4     |   blog
2         |   2     |   poll
3         |   5     |   blog

blog table
-------------------
id        |   body
-------------------
1         |   aaaa    
2         |   bbbb    
3         |   cccc
4         |   dddd
5         |   eeee

poll table
-----------------------------
id        |   text |   answer
-----------------------------
1         |   zzz  |   z   
2         |   xxx  |   x  
3         |   yyy  |   y

final output for id_company IN (1, 2) must like this:

-------------------------------------------------------
id_company|   id_act|   type|   body|   text|   answer
-------------------------------------------------------
1         |   1     |   blog|   aaaa|   NULL|   NULL
1         |   2     |   blog|   bbbb|   NULL|   NULL
2         |   3     |   blog|   cccc|   NULL|   NULL
1         |   1     |   poll|   NULL|   zzz |   z
2         |   4     |   blog|   dddd|   NULL|   NULL
2         |   2     |   poll|   NULL|   xxx |   x

Please help me to write this query.

Thanks.

Answer 1

Use left join with additional join condition on basis of type column from activity table

select 
a.id_company,
a.id_act,
a.type,
b.body,
p.text,
p.answer
from activity a 
left join blog b on(a.id_act = b.id and a.`type` = 'blog')
left join poll p on(a.id_act = p.id and a.`type` = 'poll')
where a.id_company IN (1, 2)

Demo

Answer 2

Sorry I don't have a database to hand to try this but this should work :-

SELECT * FROM 
(
SELECT 
    activity.id_company
  , activity.id_act
  , activity.type
  , blog.body
  , poll.text
  , poll.answers
FROM 
activity
LEFT JOIN blog ON activity.id_act = blog.id
LEFT JOIN poll ON activity.id_act = poll.id 
)
WHERE id_company IN (1, 2)