Oracle与DB2、MySQL取前10条记录的对比

Oralce的示例：

1. 最佳选择：利用分析函数

row_number() over ( partition by col1 order by col2 )
比如想取出100-150条记录,按照tname排序

select tname,tabtype from (
select tname,tabtype,row_number() over ( order by tname ) rn from tab
)
where rn between 100 and 150;

 

2. 使用rownum 虚列

select tname,tabtype from (
select tname,tabtype,rownum rn from tab where rownum <= 150
)
where rn >= 100;

 

注释：使用序列时不能基于整个记录集合来进行排序,假如指定了order by子句,排序的的是选出来的记录集的排序。

 

create table mynumber(id int,name varchar(10));
insert into mynumber values(1,'no1');
insert into mynumber values(2,'no2');
insert into mynumber values(3,'no3');
insert into mynumber values(4,'no4');
insert into mynumber values(5,'no5');
insert into mynumber values(5,'no6');
insert into mynumber values(6,'no7');
insert into mynumber values(7,'no8');
insert into mynumber values(8,'no9');
insert into mynumber values(9,'no10');
insert into mynumber values(9,'no11');
insert into mynumber values(9,'no12');
insert into mynumber values(10,'no13');
insert into mynumber values(10,'no14');
insert into mynumber values(10,'no15');
insert into mynumber values(11,'no16');
insert into mynumber values(12,'no17');
insert into mynumber values(13,'no18');

 

select id,name,RANK() over ( order by id ) case1,
DENSE_RANK() over ( order by id ) case2,
row_number() over ( order by id ) case3 from mynumber;

 

(1) 取前10条不同id记录，假如最后1条记录的ID依然有相同的，那么取出来。

select id,name from mynumber where id in
(select id from (select distinct id
from mynumber) tt where rownum<=10);

select * from (select id,name,RANK()
over ( order by id ) case1,DENSE_RANK()
over ( order by id ) case2,row_number()
over ( order by id ) case3 from mynumber)
tt where case1<=10;
select * from (select id,name,RANK() over
( order by id ) case1,DENSE_RANK() over
( order by id ) case2,row_number() over
( order by id ) case3 from mynumber)
tt where case1 between 5 and 10;

 

 

(2)取前10条记录，假如第10条记录的ID 还有相同的，那么取出来。

select * from mynumber where id in
(select id from mynumber where rownum <=10);

select * from (select id,name,RANK()
over ( order by id ) case1,DENSE_RANK()
over ( order by id ) case2,row_number()
over ( order by id ) case3 from mynumber)
tt where case2<=10;
select * from (select id,name,RANK()
over ( order by id ) case1,DENSE_RANK()
over ( order by id ) case2,row_number()
over ( order by id ) case3 from mynumber)
tt where case2 between 5 and 10;

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【责编:John】

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