Subquery on JOIN to pulling all ID/Names - mysql

Question

* user
user_id
name
* client
client_id
name
* user_client
user_client_id
user_id
client_id
* message
message_id
client_id
description
Sample Table Rows
user_id
1
2
3
client_id name
10 John
11 James
12 David
13 Richard
14 Bob
user_client
user_id client_id
1 11
1 13
3 14
3 10
message
message_id client_id message
1 11 Hello Word
2 12 MySQL is awesome
3 14 I like StackOverflow
4 13 This is very cool
What it's not working is when I use that query as a subquery on a LEFT JOIN to pull the messages only for those clients pertinent to the user.
Any ideas?
Thanks!

Answer 1

The DDL to set up the example (in MySQL) and the query I believe you are looking for are shown below.
/*
-- DDL TO SET UP EXAMPLE
create schema example;
use example;
create table user (
user_id int,
name varchar(64)
);
create table client(
client_id int,
name varchar(64)
);
create table user_client (
user_client_id int,
user_id int,
client_id int
);
create table message(
message_id int,
client_id int,
message varchar(64)
);
insert into user values (1, 'Peter');
insert into user values (2, 'Paul');
insert into user values (3, 'Mary');
insert into client values (10, 'John');
insert into client values (11, 'James');
insert into client values (12, 'David');
insert into client values (13, 'Richard');
insert into client values (14, 'Bob');
insert into user_client values (1, 1, 11);
insert into user_client values (2, 1, 11);
insert into user_client values (3, 3, 14);
insert into user_client values (4, 3, 10);
insert into message values (1, 11, 'Hello World');
insert into message values (2, 12, 'MySQL is awesome');
insert into message values (3, 14, 'I like StackOverflow');
insert into message values (4, 13, 'This is very cool');
*/
-- query to get all messages for all clients of a given user
select
*
from
user_client uc
join user u on uc.user_id = u.user_id
join client c on uc.client_id = c.client_id
join message m on m.client_id = c.client_id
where
u.user_id = 1;
-- query to get all messages for a given client
select
*
from
user_client uc
join user u on uc.user_id = u.user_id
join client c on uc.client_id = c.client_id
join message m on m.client_id = c.client_id
where
c.client_id = 11;

Answer 2

This should really be done as three separate queries as three different questions are being asked based on the comments:
So you want a query that will return messages from related clients if
there is a record in client, and all messages if there is no record in
client for a give user? – John
...
Hi #John that is correct. And that's why I was using the subquery,
because it pulls exactly that, but for some reason the client_id and
name are coming as NULL for all but one. – Kitara
The first question (query) is find all of the messages for all of the clients of a given user.
The second question (query) is: Does a user have "authorization" to view all messages. If the user has no clients then that user is "authorized" to view all messages.
The third question (query) is: If the user is authorized to view all messages get all of the messages.
These are very simple straight forward queries to write, execute, and understand. Trying to conflate all of this into a single query will add complexity and represents poor separation of concerns. If executing three very simple queries represents a performance issue the architecture of the application needs to be reconsidered.

Answer 3

There was a mistake in my original ddl in one of the inserts (fixed below). In the sql below I've also added a user with no messages. I believe the query at the end of what is posted below is what you are looking for.
-- DDL TO SET UP EXAMPLE
drop schema example;
create schema example;
use example;
create table user (
user_id int,
name varchar(64)
);
create table client(
client_id int,
name varchar(64)
);
create table user_client (
user_client_id int,
user_id int,
client_id int
);
create table message(
message_id int,
client_id int,
message varchar(64)
);
insert into user values (1, 'Peter');
insert into user values (2, 'Paul');
insert into user values (3, 'Mary');
insert into client values (10, 'John');
insert into client values (11, 'James');
insert into client values (12, 'David');
insert into client values (13, 'Richard');
insert into client values (14, 'Bob');
insert into client values (15, 'Quiet Client');
insert into user_client values (1, 1, 11);
insert into user_client values (2, 1, 13);
insert into user_client values (3, 3, 14);
insert into user_client values (4, 3, 10);
insert into user_client values (5, 1, 15);
insert into message values (1, 11, 'Hello World');
insert into message values (2, 12, 'MySQL is awesome');
insert into message values (4, 13, 'This is very cool');
insert into message values (3, 14, 'I like StackOverflow');
-- query to get all messages for all clients of a given user
select
u.user_id,
u.name user_name,
c.client_id,
c.name client_name,
m.message
from
user_client uc
join user u on uc.user_id = u.user_id
join client c on uc.client_id = c.client_id
left outer join message m on m.client_id = c.client_id
where
u.user_id = 1;
Output:
+ ------------ + -------------- + -------------- + ---------------- + ------------ +
| user_id | user_name | client_id | client_name | message |
+ ------------ + -------------- + -------------- + ---------------- + ------------ +
| 1 | Peter | 11 | James | Hello World |
| 1 | Peter | 13 | Richard | This is very cool |
| 1 | Peter | 15 | Quiet Client | |
+ ------------ + -------------- + -------------- + ---------------- + ------------ +
3 rows