mysql - MySQL - 每个特定 id 仅返回 1 条记录

Question

我是 sql 的新手。

我有这张表，上面有这些记录：

桌子mark

----------------------------------------
student_id | subject_id | comp_id | mark
----------------------------------------
 1         |  1         |   28    |  5.5
 1         |  1         |   29    |  7.5
 1         |  1         |   30    |  9.0
 1         |  2         |   36    |  9.6
 1         |  2         |   37    |  6.0
 1         |  3         |   42    |  4.0

我试过这个查询SELECT subject_id FROM mark WHERE student_id =1

结果

subject_id
----------
1
1
1
2
2
3

如何选择具有这些结果的数据？我期待这样的事情：

subject_id
----------
1
2
3

提前致谢。

Answer 1

只需使用DISTINCT：

SELECT DISTINCT subject_id FROM mark WHERE student_id =1

Answer 2

利用 distinct

SELECT distinct subject_id 
FROM mark 
WHERE student_id = 1

或对数据进行分组

SELECT subject_id 
FROM mark 
WHERE student_id = 1
group by subject_id

Answer 3

使用DISTINCT子句就可以实现

SELECT DISTINCT subject_id FROM mark WHERE student_id =1;

然后你会得到输出

subject_id
----------
1
2
3

Answer 4

您可以使用GROUP BY：

SELECT subject_id FROM mark WHERE student_id =1 GROUP BY student_id;