每隔N行生成一个汇总行(总结)

接前文
http://blog.itpub.net/29254281/viewspace-2149120/
http://blog.itpub.net/29254281/viewspace-2149309/

处理这种每隔N行生成一个汇总行的需求，一共有三种方式

 drop table t;
 create table t (c int);
 insert into t values
 (15),(7),(9),(10),(7),(8),(20),(16),(9),(19),
 (14),(10),(11),(10),(10),(12),(7),(10),(7),(9);
 commit;


 select * from t;

为了简单，就按照这个自然顺序 ，每三行生成一个汇总行,sum前三行的值。

第一招，效率低下容易理解

 select case when c is null then '汇总' else '' end s,ifnull(c,sumc) c from (
     select ceil(id/3) l,c,null sumc
     from
     (
         select @id:=@id+1 id, t1.* from t t1,(select @id:=0) vars
     ) t2
     union all
     select ceil(id/3) l,null,sum(c)
     from
     (
         select @id1:=@id1+1 id, t1.* from t t1,(select @id1:=0) vars
     ) t3
     group by l
 ) t order by l,ifnull(c,'9999');

结果：


第二招，数字辅助表补全. 性能较好. 计算汇总行,理解稍微复杂.

先增加一个数字辅助表 nums

 create table nums(id int not null primary key);

 delimiter $$
 create procedure pCreateNums(cnt int)
 begin
     declare s int default 1;
     truncate table nums;
     while s<=cnt do
         insert into nums select s;
         set s=s+1;
     end while;
 end $$
 delimiter ;

 delimiter $$
 create procedure pFastCreateNums(cnt int)
 begin
     declare s int default 1;
     truncate table nums;
     insert into nums select s;
     while s*2<=cnt do
         insert into nums select id+s from nums;
         set s=s*2;
     end while;
 end $$
 delimiter ;

初始化数字辅助表
call pFastCreateNums(100000);

然后

 select s,ifnull(c,cc) c
 from (
     select
     case when rn is null then '汇总' else '' end s,
     t4.c,
     if(mod(t3.id,4)!=0 ,case when @total=-1 then @total:=t4.c else @total:=@total+t4.c end,@total) cc,
     case when mod(t3.id,4)=0 then @total:=-1 else null end
     from (
         select * from nums where id<=
         (select (ceil(count(*)/4)+1)*5  from t )
     ) t3
     left join (
         select
         case when mod(@rn+1,4)=0 then @rn:=@rn+2 else @rn:=@rn+1 end rn ,
         t1.* from t t1,(select @rn:=0,@total:=0) vars
     ) t4 on(t3.id=t4.rn)
 ) result;

这个理解稍微有点复杂,
第二招改进版本

 select s,ifnull(c,cc) c
 from (
     select
     case when rn is null then '汇总' else '' end s,
     t4.c,
     case when t4.c is not null then @total:=@total+t4.c when t4.c is null then @total+(@total:=0) end cc
     from (
         select * from nums where id<=
         (select (ceil(count(*)/4)+1)*5  from t )
     ) t3
     left join (
         select
         case when mod(@rn+1,4)=0 then @rn:=@rn+2 else @rn:=@rn+1 end rn ,
         t1.* from t t1,(select @rn:=0,@total:=0) vars
     ) t4 on(t3.id=t4.rn)
 ) result where ifnull(c,cc) is not null;

好理解多了,只是有冗余行数。



第三招，又是王工的大招. 性能比第二招快15%-20%。可读性强 好理解。整体还简短。

 select case when id is null then '汇总' else '' end s,sum(c) c from (
     select @id:=@id+1 id, t1.* from t t1,(select @id:=0) vars
 ) t2
 group by ceil(t2.id/3),t2.id with rollup;

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