Question

我正在尝试从表中选择多个列，但我想根据一列选择最多一定数量的记录。我试过这个：

select roll_no ,marks as Percentage
from database
where marks in (select top (3) *
                from database
                where subject = ''
                order by marks desc) order by percentage desc

我收到错误：

当选择列表中只能指定一个表达式时子查询不是用EXISTS引入的，也不是超过指定的数字记录。

我也尝试过：

select roll_no ,marks as Percentage
from database
where marks in (select top (3) marks
                from database
                where subject = ''
                order by marks desc) order by percentage desc

为某些科目返回正确的结果，但对于其他科目，则会显示其他科目的最高分。

例如：

+---------+-------+
| roll_no | marks |
+---------+-------+
|10003    |  87   | 
|10006    |  72   | 
|10003    |  72   |    
|10002    |  67   |     
|10004    |  67   |  
+---------+-------+

如何正确构建查询？

示例数据：

+---------+-------+---------+
| roll_no | marks |subject  |
+---------+-------+---------+
|10001    |  45   | Maths   |
|10001    |  72   | Science |
|10001    |  64   | English |     
|10002    |  52   | Maths   |        
|10002    |  35   | Science |   
|10002    |  75   | English |      
|10003    |  52   | Maths   |        
|10003    |  35   | Science |   
|10003    |  75   | English | 
|10004    |  52   | Maths   |        
|10004    |  35   | Science |   
|10004    |  75   | English |  
+---------+-------+---------+

Answer 1

您可以使用窗口函数对marks列进行排名（具体为dense_rank，允许重复排名，同时保留顺序编号），然后返回排名为3或更少的所有行：

declare @t table(roll_no int identity(1,1),marks int);
insert into @t(marks) values(2),(4),(5),(8),(6),(1),(3),(2),(1),(8);

with t as
(
    select roll_no
            ,marks
            ,dense_rank() over (order by marks desc) as r
    from @t
)
select *
from t
where r <= 3;

输出：

+---------+-------+---+
| roll_no | marks | r |
+---------+-------+---+
|       4 |     8 | 1 |
|      10 |     6 | 1 |
|       5 |     6 | 2 |
|       3 |     5 | 3 |
+---------+-------+---+

Answer 2

如果我是对的并且您正在为每个主题寻找最佳的3分，那么您可以通过以下方式获得它：

DECLARE @SelectedSubject VARCHAR(50) = 'Maths'

;WITH FilteredSubjectMarks AS
(
    SELECT
        D.Subject,
        D.Roll_no,
        D.Marks,
        MarksRanking = DENSE_RANK() OVER (ORDER BY D.Marks DESC)
    FROM
        [Database] AS D
    WHERE
        D.Subject = @SelectedSubject
)
SELECT
    F.*
FROM
    FilteredSubjectMarks AS F
WHERE
    F.MarksRanking <= 3

如何从表中选择多个列时，只将TOP语句应用于1列？

2 个答案: