如何连接表，包括左表中的所有id，但只显示某些where子句的右表中的信息

Question

I have an attendees table with the following structure:

+--------------+---------+
| attendee_id  | others1 |
+--------------+---------+
|    abcd      | A       |
|    ghij      | B       |
|    defg      | C       |
+--------------+---------+

And also an eventattendees table with the following structure:

+--------------+---------+----------+
| attendee_id  | others2 | event_id |
+--------------+---------+----------+
|    wxyz      | D       |     1    |
|    mlno      | E       |     2    |
|    defg      | F       |     3    |
|    defg      | G       |     2    |
|    abcd      | H       |     1    |
+--------------+---------+----------+

What I want is to create a query that, given some event_id, returns a join of these tables (by attendee_id) that includes all attendee ids from attendee table and also returns the information from the eventattendde tables which a match for that event_id . Say, for event_id 3:

+--------------+---------+---------+----------+
| attendee_id  | others1 | others2 | event_id |
+--------------+---------+--------------------+
|    abcd      | A       |  null   |   null   |
|    ghij      | B       |  null   |   null   |
|    defg      | C       |    F    |     3    |
+--------------+---------+--------------------+

How can I do that for mysql?

Answer 1

You need to put your where criteria in the join instead to respect the outer join .

select a.attendee_id, a.others1, e.others2, e.event_id
from attendees a
   left join eventattendees e on a.attendee_id = e.attendee_id and e.event_id = 3

If you put it in the where criteria, it will negate the outer join .

Answer 2

Use left join

select a.attendee_id, a.others1, b.others2, b.event_id 
from attendees  as a 
left join eventattendees on a.attendee_id = b.attendee_id and b.event_id= 3;

Answer 3

你可以试试这个：

SELECT A.attendee_id, A.others1, ISNULL(B.others2,'empty') AS others2, ISNULL(B.event_id,'empty') AS event_id FROM TableA A LEFT JOIN TableB B ON A.attendee_id=B.attendee_id