有时候我们想要得到每个分组的前几条记录,这个时候oracle中row_number函数使用非常方便,但可惜mysql没有。网上搜了些实现方法。
表flow_task有phaseno(序列号),objectno(编号)等几个字段,我们想实现根据编号字段分组,然后组内根据序列号排序功能
select @rownum:=@rownum+1 rownum,a.objectno,a.phaseno, if(@objno=a.OBJECTNO or (@objno is null and a.objectno is null), @rank:=@rank+1, @rank:=1) as row_number, @objno:=a.OBJECTNO from(SELECT * from flow_task order by OBJECTNO,phaseno asc)a, (select @rownum :=0,@objno:=null,@rank:=0)b
注意:order by OBJECTNO,phaseno asc 分组字段在前,排序字段在后
运行结果:
原理是,先 order by OBJECTNO,phaseno asc,这样后相同编号的记录会在一块儿,并且已经是phaseno有序asc的
select的字段一个一个的看:
@rownum:[email protected]+1,每一行在上行@rownum变量值的基础上+1
if(@objno=a.OBJECTNO or (@objno is null and a.objectno is null),@rank:[email protected]+1,@rank:=1),每一行判断,当前行的objectno(编号)是否等于上一个@objno变量值,如果是在上一个@rank变量值基础上+1,否则@rank赋值1
@objno:=a.OBJECTNO,当前行objectno赋值给变量@objno
ps:如果想要分组后某个字段的几个值,也可以使用group_concat函数
select a.objectno,group_concat(ifnull(a.phaseno,‘‘)) phaseno from(SELECT * from flow_task order by OBJECTNO,phaseno asc)a GROUP BY a.objectno
运行结果:
可以看到,group_concat函数把分组后某个字段的值用,拼接起来
要获取前3个值,使用substring_index函数
select a.objectno,group_concat(ifnull(a.phaseno,‘‘)) phaseno,substring_index(group_concat(ifnull(a.phaseno,‘‘)),‘,‘,3) sub_phaseno from(SELECT * from flow_task order by OBJECTNO,phaseno asc)a GROUP BY a.objectno
扩展下:怎样实现oracle中的rank() 和dense_rank()呢?我们知道rank()排序类似:1 2 2 4...,dense_rank()排序类似:1 2 2 3...
- rank()实现:
select @rownum:=@rownum+1 rownum,a.objectno,a.phaseno, if(@objno=a.OBJECTNO or (@objno is null and a.objectno is null), if(@phaseno=a.phaseno or (@phaseno is null and a.phaseno is null),@rank,if(@sk=0,@rank:=@rank+2,@rank:=@rank+1)), @rank:=1 ) as row_number, if(@objno=a.OBJECTNO or (@objno is null and a.objectno is null), if(@phaseno=a.phaseno or (@phaseno is null and a.phaseno is null),@sk:=0,if(@sk=0,@sk:=2,@sk:=1)), @sk:=1 ) as skip, @objno:=a.OBJECTNO, @phaseno:=a.phaseno from(SELECT * from flow_task order by OBJECTNO,phaseno asc)a, (select @rownum :=0,@objno:=null,@phaseno:=null,@rank:=1,@sk:=1)b
- dense_rank()实现:
select @rownum:=@rownum+1 rownum,a.objectno,a.phaseno, if(@objno=a.OBJECTNO or (@objno is null and a.objectno is null), if(@phaseno=a.phaseno or (@phaseno is null and a.phaseno is null),@rank,@rank:=@rank+1), @rank:=1 ) as row_number, @objno:=a.OBJECTNO, @phaseno:=a.phaseno from(SELECT * from flow_task order by OBJECTNO,phaseno asc)a, (select @rownum :=0,@objno:=null,@phaseno:=null,@rank:=1)b
时间: 2024-10-05 07:52:41