如何查询两个表并从第一个表返回所有记录，而不管第二个表中是否存在

Question

I have a couple of tables with the following structure:

Table application:

AppId   Name
====================
1       App 01
2       App 02
3       App 03

Table SubscribedApplications

SubAppId   AppId    SubId
==============================
1          1        99901

What I need to get is the table application with all matches of SubscribedApplications, and if there's no record in it, get null values, filtered by the SubId. Something like this:

Expected result:

AppId   Name    SubAppId   SubId
==================================
1       App 01  1          99901
2       App 02  NULL       NULL
3       App 03  NULL       NULL

I thought of doing a right outer join like this:

select Applications.AppId as AppId,     
    Applications.Name as AppName,         
    SubscribedApplications.SubAppId as SubAppId,     
    SubscribedApplications.SubId as SubId,       
from SubscribedApplications 
    right outer join Applications on Applications.AppId = SubscribedApplications.AppId
where SubscribedApplications.SubId is null
    or SubscribedApplications.SubId= '99901' 

However, this approach is not working. If I create a record in subscribedapplications for, say, subid 99901, I get three records, but if I query for 99902 I only get two records. I can't find out why. I have tried several variants of this, including using in (null, '99901') in the where clause, to no avail.

My other alternative would be to retrieve all records from Application table, then the records from SubscribedApplication record and in (C#) code evaluate which ones to keep, but I'd like to have it in one query, if possible.

Answer 1

Move your where condition with JOIN like following.

SELECT a.AppId     AS AppId, 
       a.Name      AS AppName, 
       s.SubAppId  AS SubAppId, 
       s.SubId     AS SubId, 
FROM   SubscribedApplications  s 
       RIGHT OUTER JOIN Applications  a 
                     ON a.AppId  = s.AppId  
                        AND s.SubId = 99901 

Note: As a best practice you should use alias name for your table. I have modified your query by adding the alias name.

Answer 2

I would write this as a left join:

SELECT
    a.AppId,
    a.Name,
    sa.SubAppId,
    sa.SubId
FROM application a
LEFT JOIN SubscribedApplications sa
    ON a.AppId = sa.AppId;

Demo

Answer 3

Use left join

  Select a.*,sub.* from application a left  join 
  subscribeapplication sub a.applid=sub.appid and 
  sub.subid=99901

To get advantage of outer join you need to move where clause condition into on clause