连接两个表，其中某些列合并而其他列必须为空

Question

I feel like this question must have already been answered somewhere but my searching has not turned up anything relevant. If anyone is able to link to another answer I can accept this being marked as a duplicate and closed.

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I have three tables like this:

users(id)
things1(id, user_id, thing1_text)
things2(id, user_id, thing2_text)

For the purpose of this example, assume there's only one user in the users table with an id of 1 .

Let's say things1 looks like this:

id
1
2 user_id
1
1 thing1_text
im a thing 1
im a thing 1 too

And things2 looks like this:

id
1
2 user_id
1
1 thing2_text
im a thing 2
im a thing 2 too

I'm trying to create the query which will return this:

user_id
1
1
1
1 thing1_text
im a thing 1
im a thing 1 too
NULL
NULL thing2_text
NULL
NULL
im a thing 2
im a thing 2 too

My first, admittedly naive, attempt looked like this:

select users.id as user_id
     , things1.thing1_text
     , things2.thing2_text
from users
full outer join things1
    on things1.user_id = users.id
full outer join things2
    on things2.user_id = users.id;

However this doesn't put NULL in any of the fields and instead combines them all - which I'm sure the more SQL minded among you probably figured when you read the query :D

Anyway. This is where I'm at. Any help at all would be greatly appreciated.

Answer 1

You seem to want union all . Start with this:

select user_id, thing1_text, thing2_text
from ((select user_id, thing1_text, NULL as thing2_text
       from things1 t1 join
            users u
            on t1.user_id = u.id
      ) union all
      (select user_id, NULL, thing2_text
       from things2 t2 join
            users u
            on t2.user_id = u.id
      )
     ) tt
order by user_id;