MySQL完全连接三张表

Question

有一个问答讨论系统，其中包含三个表格，用于三个级别的问答。 表3（三级注释： GPcommentsFollowup2 ）与表2（第二级注释： GPcommentsFollowup ）相关，而表2与表1（第一级注释： GPcomments ）相关。 第2级和第3级实际上是第1级的后续注释。我想提取每个用户的贡献，并显示用户在哪个级别贡献的树。 例如：

L1-- Title:
|--L2-- Title:
    |--L3-- Title:
    |--L3-- Title:
|--L2-- Title:
    (no L3 contributed)
|--L2-- Title:
    |--L3-- Title:
    |--L3-- Title:
    |--L3-- Title:
L1-- Title:
    (no L2 and L3 contributed)

我尝试了下面的代码，但返回值为空。 有什么建议吗？

function userGroupContributions($pdo, $topicGroupId, $userId1){
$query= 'SELECT L1.commentTitle AS L1T, L1.commentId AS L1Id, L2.commentTitle AS L2T, L2.commentFollowupId AS L2Id, L3.commentTitle AS L3T, L3.commentFollowup2Id AS L3Id FROM GPcomments AS L1
        FULL OUTER JOIN GPcommentsFollowup AS L2 ON L1.commentId = L2.commentId
        FULL OUTER JOIN GPcommentsFollowup2 AS L3 ON L2.commentFollowupId = L3.commentFollowupId
        WHERE (L1.userId=? OR L2.userId=? OR L3.userId=?) AND topicSessionId=?';
$stmt = $pdo->prepare($query);
$stmt->execute(array($userId1, $userId1, $userId1, $topicGroupId));
$rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
foreach ($rows as $row) {
    $L1Id = $row['L1Id'];
    $L2Id = $row['L2Id'];
    $L3Id = $row['L3Id'];
    $L1T = htmlspecialchars_decode($row['L1T']);
    $L2T = htmlspecialchars_decode($row['L2T']);
    $L2T = htmlspecialchars_decode($row['L2T']);

    $data .= '<div>'.$L1T.'</div>';
}
return $data;

}

Answer 1

我已经用下面的代码解决了：

$query= 'SELECT L1.commentTitle AS L1T, L1.commentId AS L1Id, L1.commentsTypeId AS L1Type, L2.commentTitle AS L2T, L2.commentFollowupId AS L2Id, L2.commentsTypeId AS L2Type, L3.commentTitle AS L3T, L3.commentFollowup2Id AS L3Id, L3.commentsTypeId AS L3Type FROM (SELECT * FROM GPcomments ORDER BY submitDate DESC) AS L1
        LEFT OUTER JOIN (SELECT * FROM GPcommentsFollowup ORDER BY submitDate DESC) AS L2 ON L1.commentId = L2.commentId
        LEFT OUTER JOIN (SELECT * FROM GPcommentsFollowup2 ORDER BY submitDate DESC) AS L3 ON L2.commentFollowupId = L3.commentFollowupId
        WHERE (L1.userId=? AND L1.topicSessionId=?) OR (L2.userId=? AND L2.topicSessionId=?) OR (L3.userId=? AND L3.topicSessionId=?)
        UNION
        SELECT L1.commentTitle AS L1T, L1.commentId AS L1Id, L1.commentsTypeId AS L1Type, L2.commentTitle AS L2T, L2.commentFollowupId AS L2Id, L2.commentsTypeId AS L2Type, L3.commentTitle AS L3T, L3.commentFollowup2Id AS L3Id, L3.commentsTypeId AS L3Type FROM (SELECT * FROM GPcomments ORDER BY submitDate DESC) AS L1
        RIGHT OUTER JOIN (SELECT * FROM GPcommentsFollowup ORDER BY submitDate DESC) AS L2 ON L1.commentId = L2.commentId
        RIGHT OUTER JOIN (SELECT * FROM GPcommentsFollowup2 ORDER BY submitDate DESC) AS L3 ON L2.commentFollowupId = L3.commentFollowupId
        WHERE (L1.userId=? AND L1.topicSessionId=?) OR (L2.userId=? AND L2.topicSessionId=?) OR (L3.userId=? AND L3.topicSessionId=?)';