MySQL Query - SELECT DISTINCT ...（WHERE）問題

Question

正在苦苦尋找這個MySQL查詢。當我拿出WHERE子句時，所有結果都會顯示給用戶。所以這不是問題的PHP輸出。我很確定它在WHERE子句中，但我一直被卡住了很長時間，無法解決。

$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"); 

感謝您的幫助！

Answer 1

您不應在WHERE子句中引用來自LEFT JOINed表的列（例如，b.userID）。當你這樣做時，你隱式地把你的JOIN變成一個INNER JOIN。使這些測試成爲JOIN條件的一部分。

不這樣做：

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

做到這一點，而不是：

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

Answer 2

我已經清理了參加陳述和刪除不必要的DISTINCT（）。這給你什麼？

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