如何根据相关表的结果选择MySQL记录？

Question

I have two tables in this scenario: members and team_members. The members table is pretty self explanatory. The team members table stores the member's team information if they are a member of the team. If there is no row in the team members table that has a member_id of a user, then they are not in a team. What I want to do is get all the users that are not members of a team. Should I use left join, inner join, outer join, or just join? What would this query look like?

CREATE TABLE IF NOT EXISTS `members` (
  `member_id` int(15) NOT NULL AUTO_INCREMENT,
  `group_id` int(15) NOT NULL,
  `display_name` text NOT NULL,
  `email_address` text NOT NULL,
  `password` text NOT NULL,
  `status` tinyint(1) NOT NULL,
  `activation_code` varchar(16) NOT NULL,
  `date_joined` text NOT NULL,
  PRIMARY KEY (`member_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1;

CREATE TABLE IF NOT EXISTS `team_members` (
  `team_member_id` int(15) NOT NULL AUTO_INCREMENT,
  `member_id` int(15) NOT NULL,
  `team_id` int(15) NOT NULL,
  `date_joined` text NOT NULL,
  `date_left` text NOT NULL,
  `total_xp` int(15) NOT NULL,
  PRIMARY KEY (`team_member_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1;

Answer 1

There's several ways to write this query.

To me this is the easiest to read and understand:

select * from members where member_id not in (select member_id from team_members).

This is a really simple way to write it. If you decide you want everything you can quickly comment out the where clause:

select m.* from members m left outer join team_members tm on m.member_id = tm.member_id
where tm.member_id is null

This way doesn't seem very popular from the SQL I read but I think it's straightforward:

select m.* from members m where not exists
    (select member_id from team_members tm where tm.member_id = m.member_id)

Answer 2

On the face of it, the below query is fine

SELECT members.member_id 
FROM   members 
       LEFT OUTER JOIN team_members 
                    ON team_members.member_id = members.member_id 
WHERE  team_members.member_id IS NULL 

This will do, but on reading your question again, you seem to have a date_left column and if you want only those members who have not yet left a team then

SELECT members.member_id 
FROM   members 
       LEFT OUTER JOIN (SELECT * 
                        FROM   team_members 
                        WHERE  team_members.date_left != '') CURRENT_TEAMS 
                    ON CURRENT_TEAMS.member_id = members.member_id 
WHERE  CURRENT_TEAMS.member_id IS NULL 

SQLFiddle example http://www.sqlfiddle.com/#!2/46b25/6/0