原文:SQL Reverse函数

Sql sever里面有个自带的reverse函数，这个函数的主要功能是把一个字符产反转。比如对于：

select REVERSE('hello,world')

将得到如下的输出：dlrow,olleh

现在我的问题是，不使用这个函数而使一个字符串反转。

我找出来一种算法，第一种是使用递归，先找出最后一个字符，然后递归。最好的方式就是使用cte了。代码如下：

;with cte1(seq, vv) as(
        select 1, 'fine' union all
        select 2, 'great' union all
        select 3, 'adfinioqweiweio' union all
        select 5, 'hello,world' union all
        select 4, 'piasdf'),

cte2(seq, vv, vvs, len) as(
        select seq, vv, cast(substring(vv, len(vv), 1) as varchar(200)), len(vv)
        from cte1
)

,cte3(seq, vv, vvs, len) as(
        select *
        from cte2

        union all

        select c2.seq, c3.vv, cast(c3.vvs + substring(c3.vv, c3.len-1, 1) as varchar(200)), c3.len-1
        from cte2 c2 join cte3 c3 on
        c2.seq = c3.seq and c3.len <= c2.len and c3.len > 1
)

select * from cte3
where len = 1
order by seq, len

另外一种方式是，我先得到所有的单个字符，然后把这些字符从后往前聚合起来。代码如下：

;with cte1(id, data) as
(
        select 1, 'Jacob' union all
        select 2, 'Sebastn' union all
        select 3, 'Hello,world'
)

,Numbers AS (
    SELECT *, SUBSTRING(data, n, 1) as sub
    FROM cte1 join dbo.Number
    on N <= len(data)
)

select *
from cte1 c1 cross apply(
        select substring(sub, 1, len(sub))
        from Numbers nm
        where nm.id = c1.id
        order by n desc
        for xml path('')
) as c(p)

关于如何使用这种聚合，我将在后面的文章中详解。