Question

I have been recently going through my database code trying to improve on my old code with new techniques to make it more efficient. Recently I have been looking to JOINs and I spotted what I thought was a perfect opportunity to test it out.

I am currently using the following two queries to form a list of groups for a user:

SELECT g.group_id, g. name 
FROM assigned_groups ag, user_groups g 
WHERE ag.user_id=:user_id AND ag.group_id=g.group_id

SELECT g.group_id, g. name 
FROM users u, user_groups g 
WHERE u.user_id=:user_id AND u.base_group=g.group_id

Those two select statements give me the list but I would like to join them into one. The only catch is a user may or may not have any groups listed in “assigned_groups”. Ie thats optional.

From what I understand that means I need a LEFT or RIGHT join. I have currently the following syntax:

SELECT g.group_id, g. name 
FROM 
  user_groups g, users u 
  LEFT JOIN assigned_groups ag ON ag.user_id=:user_id AND ag.group_id=g.group_id 
WHERE 
 u.base_group=g.group_id AND u.user_id=:user_id

However this is giving me the following error:
Unknown column ‘g.group_id’ in ‘on clause’

Here is an image of my table structure:

Answer 1

Change from implicit to explicit JOIN.

This makes the g alias available later on in the FROM clause.

SELECT g.group_id, g.name, 
    ag.name -- guessing here
FROM 
  users u
  JOIN
  user_groups g ON u.base_group = g.group_id 
  LEFT JOIN 
  assigned_groups ag ON ag.user_id = :user_id AND ag.group_id = g.group_id 
WHERE 
  u.user_id = :user_id

Note: this also removes ambiguity in “which join to do first”

However, I think you really want this. It depends on the relationship between u and ag

Edit 2, after more comments

SELECT g.group_id, g.name , 1 as IsBaseGroup
FROM 
  users u
  JOIN
  user_groups g ON u.base_group = g.group_id 
WHERE 
  u.user_id = :user_id
UNION
SELECT ag.group_id, ag.name, 0 as IsBaseGroup
FROM 
  users u
  JOIN 
  assigned_groups ag ON u.user_id = ag.user_id
  JOIN
  user_groups g ON ag.group_id  = g.group_id 
WHERE 
  u.user_id = :user_id;

Edit, after comment.

SELECT 
   g.group_id, g.name, 
   CASE WHEN u.base_group = g.group_id THEN 1 ELSE 0 END AS IsBaseGroup
FROM 
  users u
  JOIN 
  assigned_groups ag ON u.user_id = ag.user_id
  JOIN
  user_groups g ON ag.group_id  = g.group_id 
WHERE 
  u.user_id = :user_id