Question

在这个MySQL查询中苦苦挣扎。当我取出WHERE子句时，所有结果都会显示给用户。所以这不是php输出的问题。我很确定它是在WHERE子句中，但我已经被困住了很长时间而无法解决它。

$test = $db->query
("SELECT DISTINCT(a.id), a.name 
FROM test AS a LEFT JOIN(testusers AS b,testclasses AS c, userclasses AS d)
ON (c.classID = d.classID AND a.id = b.testID)
WHERE d.userID='" . $_SESSION["id"] . "' OR b.userID= '" . $_SESSION["id"] . "' 
GROUP BY a.id");

感谢您的帮助！

Answer 1

您不应在WHERE子句中引用LEFT JOINed表（例如b.userID）中的列。当你这样做时，你隐式将你的JOIN变成INNER JOIN。让这些测试成为JOIN条件的一部分。

不要这样做：

/* Wrong */
SELECT *
    FROM test a
        LEFT JOIN test2users b
            ON a.id = b.testid
    WHERE b.userID = 123

请改为：

/* Right */
SELECT *
    FROM test a
        LEFT JOIN test2users b
            ON a.id = b.testid
                AND b.userID = 123

Answer 2

我清理了连接语句并删除了不必要的DISTINCT（）。这给你带来了什么？

SELECT a.id, a.name 
FROM " . $config["table_prefix"] . "test AS a
LEFT JOIN " . $config["table_prefix"] . "test2users AS b
    ON a.id = b.testID
    AND b.userID= '" . $_SESSION["id"] . "' 
LEFT JOIN " . $config["table_prefix"] . "test2classes AS c
LEFT JOIN " . $config["table_prefix"] . "test2classes AS d
    ON c.classID = d.classID
    AND d.userID='" . $_SESSION["id"] . "'
GROUP BY a.id

MySQL查询 - SELECT DISTINCT ...（WHERE）问题

2 个答案: