php - INNER JOIN 和 UNION 的问题

Question

这个问题已经有答案了:

What to do with mysqli problems? Errors like mysqli_fetch_array(): Argument #1 must be of type mysqli_result and such (1 个回答)

已关闭 6 年前。

下面的查询返回此错误:

Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given in xxx.php on line 21. Fatal error: Call to a member function fetch_assoc() on boolean in xxx.php on line 24

只有当我从两个 SELECT 语句中删除“userID”列时，该查询似乎才起作用。这与我在 INNER JOIN 中使用“userID”这一事实有什么关系吗？我该如何解决这个问题？提前致谢。

查询；

$sql = "SELECT parentID, userID, Rating, Comment, upvote_count, Time FROM     ratings_n_comments
        INNER JOIN user_details
        ON ratings_n_comments.userID=user_details.UserID 
        WHERE ratings_n_comments.mID= '".$mID."'    
        UNION
        SELECT parentID, userID, Rating, Comment, upvote_count, Time FROM replys_to_comments
        INNER JOIN user_details
        ON replys_to_comments.userID=user_details.UserID 
        WHERE replys_to_comments.mID= '".$mID."'";

Answer 1

表 user_details 和 ratings_n_comments 具有相同的列名 userID，由于不明确，因此不会显示结果。所以，使用 user_details.userID 或像我在下面使用的 select 语句中的 ratings_n_comments.userID

"SELECT parentID, user_details.userID, Rating, Comment, upvote_count, Time FROM     ratings_n_comments
        INNER JOIN user_details
        ON ratings_n_comments.userID=user_details.UserID 
        WHERE ratings_n_comments.mID= '".$mID."'    
        UNION
        SELECT parentID, user_details.userID, Rating, Comment, upvote_count, Time FROM replys_to_comments
        INNER JOIN user_details
        ON replys_to_comments.userID=user_details.UserID 
        WHERE replys_to_comments.mID= '".$mID."'";