加入两个表和group by问题

Question

我有两个表：

问题

id | name

回应

id | problem_id | user_id | value

我有下面的语句。

SELECT 
`problems`.name, 
problem_id, 
sum(value) as knowledge, 
COUNT(*) as attempts FROM `responses` 
LEFT JOIN `problems` ON `problems`.id = `responses`.problem_id 
WHERE problem_id IS NOT NULL AND user_id = 4 
GROUP BY problem_id 

它生成一个列表，像这样：

| name  | problem_id | knowledge | attempts | 
| NULL  | 1   | 6   | 6  | 
| NULL  | 2   | 5   | 6  | 
| NULL  | 3   | 4   | 6  | 
| NULL  | 4   | 3   | 5  | 

我失去了一些东西，我将不胜感激，如果有人可以帮助格式：

| name  | problem_id | knowledge | attempts | 
| Problem A | 1   | 6   | 6  | 
| Problem B | 2   | 5   | 6  | 
| Problem C | 3   | 4   | 6  | 
| Problem D | 4   | 3   | 5  | 

Answer 1

试试这个。

select p.`name`, p.`id`, r.`user_id`, sum(r.`value`) as knowledge 
from 
`responses` r 
join `problems` p on r.`problem_id` = p.`id` 
where 
r.`user_id` = 4 
group by p.`name`, p.`id` 

Answer 2

可以在没有你选择既不是聚合函数（如SUM或AVG）的东西，也不是GROUP BY的可变部分。

就你而言，名字既不是。

有些DBMS可能会让你这样做（MySQL），但结果是不可预测的。

解决方案：

SELECT 
`problems`.name, 
problem_id, 
sum(value) as knowledge, 
COUNT(*) as attempts FROM `responses` 
LEFT JOIN `problems` ON `problems`.id = `responses`.problem_id 
WHERE problem_id IS NOT NULL AND user_id = 4 
GROUP BY problem_id, problems.name