Question

我已经搜索过，但似乎无法找到我是如何做到这一点的。基本上我有两个表，如果这个稍微复杂一点，简单的连接就可以了。

我想要达到的目标是每个学生选择两个科目，并且每周为每个科目分数一定数量的分数。然后我希望能够显示学生的名字，他的科目1，然后是分数，然后是科目2和分数。通过下面的查询，这显然适用于显示主题1中的分数，但我对如何获得主题2感到困惑。

$query = "SELECT pupils.name, pupils.subject1, pupils.subject2, subjects.week1 + week2 + week3 AS subject1points 
            FROM pupils, subjects 
            WHERE pupils.subject1 = subjects.subject";

$result = @mysql_query ($query);

if ($result) {
    echo....

也许是某种子查询？提前致谢。当然，我正在完成桌面并显示结果。

Answer 1

您可以加入subjects三次，但您需要使用别名来消除列的歧义：

SELECT 
    pupils.name, 
    pupils.subject1, 
    pupils.subject2,
    pupils.subject3,
    s1.week1 + s1.week2 + s1.week3 AS subject1points,
    s2.week1 + s2.week2 + s2.week3 AS subject2points,
    s3.week1 + s3.week2 + s3.week3 AS subject3points
FROM 
    pupils, subjects s1, subjects s2, subjects s3
WHERE 
    pupils.subject1 = s1.subject
    AND
    pupils.subject2 = s2.subject
    AND
    pupils.subject3 = s3.subject

在上面的查询中，s1，s2和s3是别名。定义后，您可以在通常使用完整表名的任何地方使用它们。

另一种方式，也就是我个人的偏好，是使用JOIN语法：

SELECT 
    p.name, 
    p.subject1, 
    p.subject2,
    p.subject3,
    s1.week1 + s1.week2 + s1.week3 AS subject1points,
    s2.week1 + s2.week2 + s2.week3 AS subject2points,
    s3.week1 + s3.week2 + s3.week3 AS subject3points
FROM 
    pupils p
JOIN
    subjects s1 ON p.subject1 = s1.subject
JOIN
    subjects s2 ON p.subject2 = s2.subject
JOIN
    subjects s3 ON p.subject3 = s3.subject

在上面的查询中，我还将pupils别名为p。我更喜欢这种语法，因为它非常清楚。如果任何学生的科目可以为空（subject1，subject2或subject3），那么您可能需要将这些更改为LEFT JOIN以便您仍然可以查看任何非空科目的结果。

Answer 2

使用三个连接。我想这就是你想要的：

SELECT p.name, p.subject1, (s1.week1 + s1.week2 + s1.week3) as score1,
       p.subject2, (s2.week1 + s2.week2 + s2.week3) as score2,
       p.subject3, (s3.week1 + s3.week2 + s3.week3) as score3
FROM pupils p LEFT JOIN
     subjects s1
     ON p.subject1 = s1.subject LEFT JOIN
     subjects s2
     ON p.subject2 = s2.subject LEFT JOIN
     subjects s3
     ON p.subject3 = s3.subject;

请注意，您应该学习使用正确的显式join语法，尤其是在您相对较新的SQL时。现在是开始养成良好习惯的时候了。

Answer 3

我会查询加入一张有两份主题表的学生表。

修改
我不打扰我所表达的想法，因为其他人已经这样做了。但是，只有2个科目，而不是3个（分数为3周），但这只是意味着离开s3。

加入和两个可能的where子句

3 个答案: