Question

我有下表：

CREATE TABLE Source_Table
    ([Student_ID] varchar(10), [Class_ID] varchar(10), [Sport] varchar(10))
;

INSERT INTO Source_Table
    ([Student_ID], [Class_ID], [Sport])
VALUES
    ('S00001', 'C0123A', 'Football'),
    ('S00002', 'C0123A', 'Football'),
    ('S00003', 'C0123A', 'Football'),
    ('S00004', 'C0123A', 'Football'),
    ('S00005', 'C0111B', 'Basketball'),
    ('S00006', 'C0111B', 'Basketball'),
    ('S00007', 'C0211C', 'Basketball'),
    ('S00008', 'C0100D', 'Soccer'),
    ('S00009', 'C0100D', 'Soccer');

，我想要以下输出：

规则是：对于具有count（Student_ID）> = 3的Class_ID，仅计数一次

Sport   Count
Basketball  3
Football    **1**
Soccer  2

我尝试了GROUP BY Sport和COUNT（CLASS_ID），但不确定如何执行该规则。

http://www.sqlfiddle.com/#!18/3e6a3/3

select Sport, Count(Class_Id) from Source_Table
group by Sport

我尝试过的输出是：

Sport   Count
Basketball  3
Football    **4** when I would like it to be Football **1**
Soccer  2

Answer 1

您可以通过此查询获得所需的结果。它使用子查询为每种Sport / Class_Id组合计算学生人数。在外部查询中，它对计数进行求和，将具有3个或更多学生的班级的计数替换为1：

select Sport, SUM(CASE WHEN Count >= 3 THEN 1 ELSE Count END) AS Count
FROM (select Sport, Class_Id, Count(Student_Id) AS Count
      from Source_Table
      group by Sport, Class_Id) s
GROUP BY Sport

输出：

Sport       Count
Basketball  3
Football    1
Soccer      2

Updated SQLFiddle

Answer 2

我认为最简单的方法是case表达式：

select Sport,
       (case when count(*) > 3 then 1 else count(*) end) as cnt
from Source_Table
group by Sport;

如果您需要计算不同的学生，则可以这样做：

select Sport,
       (case when count(distinct class_id) > 3 then 1 else count(*) end) as cnt
from Source_Table
group by Sport;

有条件的COUNT和GROUP BY？

2 个答案: