Question

这个SQL查询出了什么问题：

$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";

$ get_sum是从另一个表派生的sql语句数组：

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

上面的查询返回这些值：

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

它应该返回：

 Subtotal for Age 20        1
 Subtotal for Age 24        2

在我的样本数据中，有3名受访者2名是24岁，另一名是20岁。我想总计每个年龄段的受访者人数。

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子句中包含与age具有多对一关系的任何列。因此，我删除了res.gender和answers.*列。你想要的是计数（1）组（因为你按res.Age分组）。

MySQL GROUP BY和JOIN

1 个答案: