sql - 想要在现有的选择语句中再添加两列

Question

我有一个示例查询

select 
    x as 1, y as 2 , z as 3 
from 
    table abc, xyz
where 
    a = x and x = 123

现在我想在这个 SELECT 语句中再添加两列，如下所示:

1. 第 4 列将显示行序号。
2. 第 5 列:此列将取决于行 - 它会在第一行显示 start，在最后一行显示 Last，在最后一行显示 Middle在中间的任何行中。

请建议执行此操作的最佳优化方法。

谢谢

Answer 1

除非您指定，否则数据没有顺序。

select 
    x as 1, 
    y as 2 , 
    z as 3 ,
    row_number() over (order by whatever),
    case when row_number() over (order by whatever) = 1 then 'first' 
    else
        case when row_number() over (order by whatever desc) = 1 then 'last' 
        else 'middle' 
        end
    end
from table abc 
    inner join xyz on a = x 
where x= 123

请注意在上述查询中使用 ANSI-92 连接而不是 where 子句。

您可以使用公用表表达式进一步优化它

;with cte as 
(
    select 
        x , 
        y , 
        z  ,
        row_number() over (order by whatever) rn
    from table abc 
        inner join xyz on a = x 
    where x= 123
)
    select x,y,z,rn,
        case rn when 1 then 'first'
        when (select MAX(rn) from cte) then 'last'
        else 'middle'
    end
    from cte

或者没有像这样的 CTE:

select 
    x as 1, 
    y as 2 , 
    z as 3 ,
    row_number() over (order by whatever),
    case row_number() over (order by whatever)
        when 1 then 'first' 
        when count(*) over () then 'last' 
        else 'middle' 
    end
from table abc 
    inner join xyz on a = x 
where x= 123