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;

(3)取前10条记录

select id,name from mynumber 
where rownum <=10;
select id,name from 
(select id,name,rownum rn from 
mynumber where rownum <= 10 ) where rn >= 5;

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 case3<=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 case3 between 5 and 10;

Db2示例

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 * 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) as tt where case1<=10

(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，那么全部取出来。

select * from mynumber where id in
(select distinct id from mynumber fetch first 10 rows only)

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) 
as 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) 
as 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，那么全部取出来。

select * from mynumber where id in
(select id from mynumber fetch first 10 rows only)

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) 
as 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) 
as tt where case2 between 5 and 10

(3)取前10条记录

select id from mynumber fetch first 10 rows only

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) as tt where case3<=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) 
as tt where case3 between 5 and 10

Mysql示例：

select id from mytable order by update_date desc limit 0,10

（责任编辑：卢兆林）