从3个表中选择和匹配数据

Question

SCENARIO

x3 tables:

Members

    member_id | member_name
    -------------
    1  | member 1
    2  | member 2
    3  | member 3

teams

    team_id | team_name
    -------------
    1  | team 1
    2  | team 2
    3  | team 3

team_members

    team_member_id | member_id | team_id
    ------------------------------------
    1              |  1        | 1
    2              |  2        | 2
    3              |  3        | 3

Members can be associated with more than one team so that is why I separated the data out into 3 tables.

I want to list out all the teams, and underneath them show the members of each of those teams. I am not sure what the best approach is on this one as I am new to database design.

Currently all I am doing is outputting the two values:

<?php
$sql = "SELECT * FROM team_members";
    $result = $conn->query($sql);

    if($result->num_rows > 0){
        while($row = $result->fetch_assoc()){

            echo 'teamID: <strong>' . $row["team_id"] . '</strong><br>';
            echo 'memberID: <strong>' . $row["member_id"] . '</strong><br>';

        }
    }

?>

Answer 1

You will have to JOIN between those tables like below, performing an inner join which will give only matching records. In case you want all teams irrespective of whether those teams have any member or not then consider doing a left join instead.

select t.team_id, m.member_ID, m.member_name
from teams t
left join team_mambers tm on t.team_id = tm.team_id
left join members m on m.member_ID = tm.member_ID;

Explanations:

Since you have a many to many relationship between teams and members; you have a relationship table where you maintain the relationship (OR Foreign keys of the tables) in a different table team_mambers . Thus for you to join teams with members ; you will first have to join with the relationship table and then join with the other table members based on the matched id's.

I have perform a outer join keeping in mind that not all teams may have members in it and thus outer join will get you all the teams irrespective of members in it or not.