按组查询中的最高值

Question

我知道如何获得最高价值,但我遇到了一些非常简单的问题.

我有一张学生桌.它有:

• 名称
• numberoflaps
• 年级

我希望得到一个查询或报告,显示每个等级最多圈数的前两个孩子.

Answer 1

使用MySQL:

MySQL没有任何排名功能,但它确实允许变量创建和更新:

SELECT x.grade,
       x.name,
       x.numberoflaps
  FROM (SELECT s.grade,
               s.name,
               s.numberoflaps,
               CASE 
                 WHEN @grade != s.grade THEN @rownum := 1 
                 ELSE @rownum := @rownum + 1
               END AS rank,
               @grade := s.grade
          FROM STUDENTS s,
               (SELECT @rownum := 0, @grade := NULL) r
      ORDER BY s.grade, s.numberoflaps DESC) x
 WHERE x.rank <= 2
ORDER BY x.grade, x.rank

该ORDER BY子查询是非常重要的,否则排名将无法正常进行.

使用Oracle 9i +/SQL Server 2005+:

使用CTE:

WITH laps AS (
  SELECT s.grade, 
         s.name, 
         s.numberoflaps,
         ROW_NUMBER() OVER (PARTITION BY grade ORDER BY numberoflaps DESC) AS rank
    FROM STUDENTS s)
  SELECT l.grade, 
         l.name, 
         l.numberoflaps
    FROM laps l
   WHERE l.rank <= 2
ORDER BY l.grade, l.numberoflaps DESC

非CTE等效物:

  SELECT l.grade, 
         l.name, 
         l.numberoflaps
    FROM (SELECT s.grade, 
                 s.name, 
                 s.numberoflaps,
                 ROW_NUMBER() OVER (PARTITION BY grade ORDER BY numberoflaps DESC) AS rank
            FROM STUDENTS s) l
   WHERE l.rank <= 2
ORDER BY l.grade, l.numberoflaps DESC

警告:

Oracle在9i中获得了排名功能; 对于SQL Server,它是2005年.