How to select SQL results based on multiple tables

问题

I need to select results from one table based on certain matching values in a couple of other tables. I have the following tables:

person: id, firstname, lastname
team: id, teamname
player: id, person_id(FK), team_id(FK)
coach: id, person_id(FK), team_id(FK)

I need to return all the coaches and players names for each team. I've only ever used inner joins, and it doesn't seem like I can use those here, so any idea how to do this?

回答1:

This will give you the coach:

SELECT team.Teamname, person.Firstname, person.Lastname
FROM person
JOIN coach ON person.id = coach.person_id
JOIN team  ON coach.team_id = team.id

And this will give you the players:

SELECT team.Teamname, person.Firstname, person.Lastname
FROM person
JOIN player ON person.id = player.person_id
JOIN team  ON player.team_id = team.id

So, the non-elegant, simple answer is to just toss it all together with UNION.

回答2:

Just use an OR in the join to Team

SELECT
     P.firstname,
     P.lastname,
     T.teamname
FROM
    person p id
    LEFT JOIN player pl
    ON p.id = pl.person_id
    LEFT JOIN coach c
    ON p.id = c.person_id
    LEFT JOIN team t
    ON pl.team_id = t.id
         or.c.team_id = t.id

Or if you perfer if and your database has COALESCE

   LEFT JOIN team t
        ON COALESCE(pl.team_id,c.team_id)  = t.id

来源：https://stackoverflow.com/questions/6587514/how-to-select-sql-results-based-on-multiple-tables