2015.12.24(圣诞节) 解决Oralce数据库将具有相同属性的多行合并为一行的简单方法多年想要wmsys.wm_concat

用到Oralce10g以后增加的函数wmsys.wm_concat

例如这张表的有两个字段，要按airport_id合并成两行可用sql语句

select airport_id,   wmsys.wm_concat( account) from AIRPORT_MODIFY group by airport_id

结果为：

非常简单，美中不足的是合并结果没有排序，解决排序问题最简单的办法就是在合并列前加上distinct

此外，如果合并列结果出现乱码，需要在合并列前加上to_char(),如wmsys.wm_concat( to_char(account)),据说是因为该函数在不同的Oracle版本中会返回varchar2或CLob两种结果

2016.8.15补充，关于排序

按wmsys.wm_concat(distinct account)排序只能实现按account文本排序，无法实现按某复杂逻辑排序。下面是实现方法

有普通sql如下

select flt,srtnum,seg,tran_level,name

from flight_tran_direction

where seg>0

order by flt,seg,srtnum

结果为：列举所有航线中，高度层反配的所有点

要实现按flt，seg分组并按srtsum排序的各点串，可写sql为

select flt,seg,tran_level,

WMSYS.WM_CONCAT(name) OVER(PARTITION BY flt,seg ORDER BY srtnum) KEY

from flight_tran_direction

where seg>0

order by flt,seg

结果为：类似于group by flt,seg实现了按flt,seg分组,且name按srtnum逻辑排序。唯一不足的是一个分组会多出若干历史行。实际上只需要每组最后一行。

改进sql为：外面再嵌套一层groupby结构，通过max实现取最多的合并列

select flt,seg,max(key) rt

from

(

select flt,seg,tran_level,

WMSYS.WM_CONCAT(name) OVER(PARTITION BY flt,seg ORDER BY srtnum) KEY

from flight_tran_direction

where seg>0

order by flt,seg

)

group by flt,seg,tran_level

结果为：

甚至反过按合并列rt分组，组合flt

select rt,tran, WMSYS.WM_CONCAT( distinct flt)

from

(

select flt,seg,

case mod(to_number(regexp_substr(tran_level,'^[[:digit:]]+')),2) when 1 then '西单' else '东双' end tran,

replace(max(key),',','-') rt

from

(

select flt,seg,tran_level,

WMSYS.WM_CONCAT(name) OVER(PARTITION BY flt,seg ORDER BY srtnum) KEY

from flight_tran_direction

where seg>0

order by flt,seg

)

group by flt,seg,tran_level

)

group by rt,tran

order by rt,tran