所以我有这个问题，从3个不同的表中打印出表数据作为矩阵。在搜索互联网后，我粗心地找到了解决问题的方法。但我不知道，这个mysql语句是如何工作的。 This is a example that i want to do
我认为我找到了一个解决方案，如果有人可以帮我解释一下它是如何工作的？
SELECT names.codename,
s1.score AS "Score1", s1.comment AS "Comments1",
s2.score AS "Score2", s2.comment AS "Comments2",
SUM(st.score) AS "Total"
FROM students names
LEFT JOIN scores s1 ON s1.act_id=1 AND names.id=s1.student_id
LEFT JOIN scores s2 ON s2.act_id=2 AND names.id=s2.student_id
LEFT JOIN scores st ON names.id=st.student_id
WHERE names.codename <> ''
GROUP BY names.codename
ORDER BY names.codename;
students table:
+----+---------------+
| id | codename |
+----+---------------+
| 1 | Budy |
+----+---------------+
assignments table:
+--------+------------+
| act_id | name |
+--------+------------+
| 1 | Activity 1 |
| 2 | Activity 2 |
+--------+------------+
scores table:
+------------+--------+-------+
| student_id | act_id | score |
+------------+--------+-------+
| 1 | 1 | 10 |
| 1 | 2 | 10 |
+------------+--------+-------+
现在的问题是，我希望在顶部列出作业，并在名称旁边显示分数。像这样：
+---------------+------------+------------+-------+
| codename | Activity 1 | Activity 2 | Total |
+---------------+------------+------------+-------+
| budy | 10 | 10 | 20 |
+---------------+------------+------------+-------+