MySQL INNER JOIN 3个不同的表与WHERE

Question

Sorry if this question might seem weird, but I am trying to INNER JOIN some tables in SQL and I can't make it right. I am new to this, so please if you care to explain what I'm doing wrong... i would really appreciate it.

Basically, what I am trying to do is JOIN the following tables: meetings, users and service. Users can be moderators or admins and that information is defined in the settings column. I want to INNER JOIN moderator_id with users.id WHERE users.settings = 'moderator' and also INNER JOIN admin_id with users.id WHERE users.settings = 'admin'.

Hope you guys understand what I am trying to do :)

Thanks!

SELECT meetings.*
     , users.id
     , users.name
     , users.settings
     , services.id
     , services.service_name
     , services.price 
  FROM meetings 
  JOIN users 
    ON meetings.moderator_id = users.id 
 WHERE users.settings = 'moderator' 
  JOIN users 
    ON meetings.admin_id = users.id 
 WHERE users.settings = 'admin' 
  JOIN services 
    ON meetings.service_id = services.id 
 ORDER 
    BY meetings.date ASC   

Answer 1

SELECT meetings.*, users.id, users.name, users.settings, services.id, services.service_name, services.price 
FROM meetings

INNER JOIN users as mods ON meetings.moderator_id = mods.id 
INNER JOIN users as adms ON meetings.admin_id = adms.id
INNER JOIN services ON meetings.service_id = services.id

WHERE adms.users.settings = 'admin' AND mods.users.settings = 'moderator' 
ORDER BY appointments.date ASC

I guess you need to specify the users instance for admins and moderatores, then place the where after the joins. Try this query and tell me if it worked.

Answer 2

The syntax for SELECT is:

SELECT <fields>
FROM <table>
JOIN <table> ON <condition>
JOIN <table> ON <condition>
...
WHERE <condition> AND <condition> AND ...
ORDER BY <column>, <column>, ...

ie you list all tables involved followed by the conditions. That's the general structure for an SQL query so feel free to memorize it :). Also please note that this is of course simplified in the extreme, there's much more to it. (Eg OR instead of AND in the WHERE clause, but that's another story.) Good luck.

Edit: If you alias a table, eg "FROM tablename AS newtablename" you should use the alias, newtablename, for conditions in WHERE, columns in SELECT, etc. Otherwise you get messages about things that can't be found. You sort of rename the table for the duration of the query and throw the old name away.

Answer 3

Eg:

SELECT m.*
     , mods.name mod_name
     , admins.name admin_name
     , s.service_name
     , s.price 
  FROM meetings m
  JOIN users mods
    ON mods.id = m.moderator_id 
  JOIN users admins
    ON admins.id = m.admin_id
  JOIN services s
    ON s.id = m.service_id 
 WHERE mods.settings = 'moderator' 
   AND admins.settings = 'admin';

... or, more likely...

SELECT m.*
     , mods.name mod_name
     , admins.name admin_name
     , s.service_name
     , s.price 
  FROM meetings m
  LEFT
  JOIN users mods
    ON mods.id = m.moderator_id 
   AND mods.settings = 'moderator' 
  LEFT
  JOIN users admins
    ON admins.id = m.admin_id
   AND admins.settings = 'admin'
  JOIN services s
    ON s.id = m.service_id;

Answer 4

I found the answer by combining some of your suggestions! It seems to be working just fine now, so thank you guys for your help!

SELECT meetings.*, 
  mods.id, 
  mods.name as mods_name, 
  mods.settings, 
  admins.id, 
  admins.name as admins_name, 
  admins.settings, 
  services.id, 
  services.service_name, 
  services.price

FROM meetings 

INNER JOIN users as mods ON meetings.moderator_id = mods.id 
INNER JOIN users as admins ON meetings.admin_id = admins.id
INNER JOIN services ON meetings.service_id = services.id

WHERE mods.settings = 'moderator' AND admins.settings = 'admin' 
ORDER BY meetings.date ASC