How to write a proper If...Else Statement with JOIN in MySQL? - mysql

Question

I'm quite a beginner in MySQL I just know the totally basic statements, however now I'ts time for me to get into some more difficult, but worth stuff.
I actually have 3 tables in MySQL, here is the representation:
users:
user_id | name | country
---------------------------
1 | Joseph | US
2 | Kennedy | US
3 | Dale | UK
admins:
admin_id | name | country
----------------------------
1 | David | UK
2 | Ryan | US
3 | Paul | UK
notes:
id | n_id | note | comment | country | type | manager
----------------------------------------------------------------
1 | 3 | This is the 1st note | First | US | admin | 2
2 | 2 | This is the 2nd note | Second | US | user | 1
3 | 2 | This is the 3rd note | Third | UK | user | 2
Now I would like to execute something like this SQL (I'm going to type not real commands here, because I'm not really familiar with all of the SQL expressions):
IF notes.type = admin
THEN
SELECT
notes.note,
notes.comment,
notes.country,
admins.name,
admins.country
FROM notes, admins
WHERE notes.n_id = admin.admin_id
ELSEIF notes.type = 'user'
SELECT
notes.note,
notes.comment,
notes.country,
users.name,
users.country
FROM notes, users
WHERE notes.n_id = users.user_id
I hope you understand what would I like to achieve here. I could do this easily with more SQL statements, but I would like to try some query which doesn't use that much resources.
Edit 1:
I would like to Get all of the Notes and get which usergroup has submitted it than apply the user's name to it. I mean, if the admin submitted the note, than SQL should choose the ID from the Admin table (as per the type value) but if a User submitted the note, it should get the name from the Users table.
The result should look something similar to this:
result:
------
id | note | comment | country | name
--------------------------------------------------------
1 | This is the 1st note | First | US | Paul
2 | This is the 2nd note | Second | US | Kennedy
3 | This is the 3rd note | Third | UK | Kennedy
Edit 2:
I have actually forgot to mention, that all of these should be listed to a manager. So a 'manager ID' should be added to the Notes and list all of the notes where the manager is for example: 2.

Answer 1

Here is a method that you can do in one query:
SELECT n.note, n.comment, n.country,
coalesce(a.name, u.name) as name, coalesce(a.country, u.country) as country
FROM notes n left join
admins a
on n.n_id = a.admin_id and n.type = 'admin' left join
users u
on n.n_id = u.user_id and n.type = 'user';
This uses left join to bring the records together from both tables. It then chooses the matching record for the select.
To select a particular manager, remove the semicolon and add:
where n.manager = 2;

Answer 2

If you expect admins and users in one result you have got several options. The simplest way is to make a union select like this:
SELECT
notes.note,
notes.comment,
notes.country,
admins.name,
admins.country
FROM
notes join admins on notes.n_id = admin.admin_id
WHERE
notes.manager = 2
UNION ALL
SELECT
notes.note,
notes.comment,
notes.country,
users.name,
users.country
FROM
notes join users on notes.n_id = users.user_id
WHERE
notes.manager = 2