SQL Server：以列值为条件的不同聚合函数

Question

我有一个这样的表（大大简化了）：

|student_id|Courses| grades |
+----------+-------+--------+
|    001   |  Math |   95   |
|    001   |  Math |   83   |
|    001   | Others|   33   |
|    002   |  Math |   92   |
|    002   | Others|   12   |
|    002   | Others|   72   |

我想要什么：

1. “数学”计数
2. “其他”的最低要求
3. 按Student_id分组

所需结果：

|student_id|Num_math|min_others|
+----------+--------+----------+
|    001   |    2   |    33    |
|    002   |    1   |    12    |

Answer 1

就像戈登所说，您需要使用GROUP BY，COUNT()，CASE和MIN。这就是您想要的：

SELECT student_id
        ,COUNT(CASE WHEN Courses='Math' THEN grades ELSE NULL END) Math
        ,MIN(CASE WHEN Courses='Others' THEN grades ELSE NULL END) Others
FROM Student
GROUP BY student_id

Answer 2

使用GROUP BY student_id和条件SUM和MIN：

   SELECT 
    student_id, 
    SUM(CASE Courses WHEN 'Math' THEN 1 ELSE 0 END) AS  Num_math,
    MIN(CASE Courses WHEN 'Others' THEN grades ELSE NULL END) AS min_others
   FROM tablename
    GROUP BY student_id