查询每门学科成绩在前2位的记录，并按学科，成绩排序

Create Table StuAchievement 
(
      StuId varchar(8),
      CourseId varchar(10),
      Achievement int
)

SELECT * FROM StuAchievement A
WHERE EXISTS(
        SELECT TOP 1 * FROM StuAchievement b 
        WHERE a.CourseId=b.CourseId AND b.Achievement >a.Achievement 
        HAVING COUNT(0)<=1
)
ORDER BY a.CourseId,a.Achievement DESC

SELECT * FROM StuAchievement A
WHERE NOT EXISTS(
        SELECT TOP 1 * FROM StuAchievement b 
        WHERE a.CourseId=b.CourseId AND b.Achievement >a.Achievement 
        HAVING COUNT(0)>=2
)
ORDER BY a.CourseId,a.Achievement DESC

--运用CTE
WITH temp AS (
    SELECT StuId,CourseId,ROW_NUMBER() OVER (PARTITION BY CourseId ORDER BY CourseId,Achievement DESC) AS rowId
    FROM StuAchievement
)
SELECT * FROM StuAchievement a,temp b 
WHERE a.StuId=b.StuId AND a.CourseId=b.CourseId AND b.rowId<=2
ORDER BY a.CourseId,a.Achievement DESC