数据表数据：
id courseid price
1 2 4800
2 2 2500
3 2 3000
4 3 4000
5 3 1000
6 3 5000
7 3 2500
8 3 1000
SELECT * FROM (
SELECT ID,CourseID,ci.Price,ROW_NUMBER() OVER ( partition by courseID order BY ci.Price) AS rownum
FROM foreach ci
) AS tbl WHERE rownum = 1 ORDER BY tbl.ID
结果：
ID,CourseID,ci.Price,rownum
2 2 2500 1
8 3 1000 1
