MySQL的GROUP BY和加入

Question

傢伙有什麼不對的SQL查詢：

$sql = "SELECT 
    res.Age, 
    res.Gender, 
    answer.*, 
    $get_sum, 
    SUM(CASE WHEN res.Gender='Male' THEN 1 else 0 END) AS males, 
    SUM(CASE WHEN res.Gender='Female' THEN 1 else 0 END) AS females 
    FROM Respondents AS res 
    INNER JOIN Answers as answer 
     ON answer.RespondentID=res.RespondentID 
    INNER JOIN Questions as question 
     ON answer.Answer=question.id 
    WHERE answer.Question='Q1' 
    GROUP BY res.Age 
    ORDER BY res.Age ASC"; 

的$ get_sum是SQL語句的排列從另一個表導出：

$sum[]= "SUM(CASE WHEN answer.Answer=".$db->f("id")." THEN 1 else 0 END) AS item".$db->f("id"); 
$get_sum = implode(', ', $sum); 

以上返回查詢這些值：

Age: 20  
    item1 0 
    item2 1 
    item3 1 
    item4 1 
    item5 0 
    item6 0 
Subtotal for Age 20  3 

Age: 24  
    item1 2 
    item2 2 
    item3 2 
    item4 2 
    item5 1 
    item6 0 
Subtotal for Age 24  9 

它應該返回：

Subtotal for Age 20  1 
Subtotal for Age 24  2 

在我的樣本數據中有3位受訪者，其中2位24歲，另一位20歲。 我想每個年齡的受訪者總數。

Answer 1

$sql = "SELECT 
    res.Age, 
    COUNT(1) AS SubTotalRespondentsByAge 
    $get_sum, 
    SUM(CASE WHEN res.Gender='Male' THEN 1 else 0 END) AS males, 
    SUM(CASE WHEN res.Gender='Female' THEN 1 else 0 END) AS females 
    FROM Respondents AS res 
    INNER JOIN Answers as answer 
     ON answer.RespondentID=res.RespondentID 
    INNER JOIN Questions as question 
     ON answer.Answer=question.id 
    WHERE answer.Question='Q1' 
    GROUP BY res.Age 
    ORDER BY res.Age ASC" 

您不能在select子句中包含與年齡具有多對一關係的任何列。我已經刪除了res.gender和answers.*列。你想要的是計數（1）的組（因爲你按res.Age分組）。