SQL查询FROM变量表

Question

I have 4 tables : one with events , one with those events' attendants , and two others containing information about those attendants, might they be single persons or groups of persons. The design of the database seems fair, right ?

I then need to get the name of who is going to attend a particular event. I'll then get the type of the event's attendants, and its id using a regular join. But i also need attendants' names , and its stored in two different tables depending on what type the attendant is.

Have a look at what i did :

SELECT
    ep.type_attendant 'type',
    ep.id_attendant 'id',
    IF( ep.type_attendant = 'user', CONCAT( u.firstname, ' ', u.lastname ), g.name ) 'fullname'
FROM
    events_attendants ep,
    events e,
    users u,
    groups g
WHERE
    ep.id_event = e.id AND
    ep.id_attendant = IF( ep.type_attendant = 'user', u.id, g.id )

That works, but is not perfect since it returns duplicate rows for the table groups . What i'd like to end up with is something like that : (except the one below doesn't work)

SELECT
    ep.type_attendant 'type',
    ep.id_attendant 'id',
    IF( ep.type_attendant = 'user', CONCAT( u.firstname, ' ', u.lastname ), g.name ) 'fullname'
FROM
    events_attendants ep,
    events e,
    IF( ep.type_attendant = 'user', users u, groups g ) -- variable table
WHERE
    ep.id_event = e.id AND
    ep.id_attendant = IF( ep.type_attendant = 'user', u.id, g.id )

I could also run two queries, and merge their results with PHP, but i'm the kind of guy who likes to learn.

MySQL database btw. Thanks for your help!

Answer 1

    SELECT
        ep.type_attendant AS `type`,
        ep.id_attendant   AS id,
        CONCAT( u.firstname, ' ', u.lastname ) AS fullname
    FROM
        events_attendants ep
      JOIN
        events e    ON ep.id_event = e.id 
      JOIN
        users u     ON ep.id_attendant = u.id
    WHERE
        ep.type_attendant = 'user'
UNION ALL
    SELECT
        ep.type_attendant,
        ep.id_attendant,
        g.name  
    FROM
        events_attendants ep
      JOIN
        events e    ON ep.id_event = e.id 
      JOIN
        groups g    ON ep.id_attendant = g.id
    WHERE
        ep.type_attendant <> 'user'