ORACLE和SYBASE数据库中实现数据查询条数限制的SQL语句实现
一、概述
对于某些需要通过数据库与大量数据打交道的软件来说，处理性能相当的重要。为了保证软件能够将所有数据处理完而不至于崩溃，分批处理的思想应运而生。分批处理的具体做法是编写SQL语句，每次返回规定条数的数据给软件处理，待这一批数据处理完之后，再接着处理下一批。
本文通过对具体的数据库表(tb_employeeinfo)的操作过程，展示了ORACLE和SYBASE数据库中分批处理SQL语句的编写方法。
二、ORACLE数据库中的处理
首先，建立tb_employeeinfo表，其定义如下：
begin
execute immediate "drop table tb_employeeinfo CASCADE CONSTRAINTS";
EXCEPTION WHEN OTHERS THEN NULL;
end;
/
create table tb_employeeinfo
(
employeeno varchar2(20) not null, -- no. of employee
employeename varchar2(20) not null, -- name of employee
employeeage int not null -- age of employee
);
create unique index idx1_tb_employeeinfo on tb_employeeinfo(employeeno);
prompt "create table tb_employeeinfo ok";
commit;
接着，在tb_employeeinfo表中插入7条数据，如下：
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1000", "ZhangSan", 20);
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1001", "LiSi", 21);
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1002", "WangWu", 21);
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1003", "ZhouLiu", 22);
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1004", "SunQi", 22);
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1005", "LiuBa", 23);
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1006", "ChenShi", 25);
如果我们想要一次性从tb_employeeinfo表中查询出5条数据，该如何处理呢？
ORACLE数据库中有一个rownum用在查询(select)语句中来限制每次执行之后返回的数据条数。例如，本次要从tb_employeeinfo表中返回5条数据，则编写SQL语句如下：
select employeeno, employeename, employeeage from tb_employeeinfo where rownum<=5;执行结果如下：
SQL> select employeeno, employeename, employeeage from tb_employeeinfo where rownum<=5;
EMPLOYEENO EMPLOYEENAME EMPLOYEEAGE
A1000 ZhangSan 20
A1001 LiSi 21
A1002 WangWu 21
A1003 ZhouLiu 22
A1004 SunQi 22
三、SYBASE数据库中的处理
首先，建立tb_employeeinfo表，其定义如下：
if exists(select * from sysobjects where name="tb_employeeinfo")
drop table tb_employeeinfo
go
create table tb_employeeinfo
(
employeeno varchar(20) not null, -- no. of employee
employeename varchar(20) not null, -- name of employee
employeeage int not null -- age of employee
)
go
create unique index idx1_tb_employeeinfo on tb_employeeinfo(employeeno)
go
print "create table tb_employeeinfo ok"
go
接着，在tb_employeeinfo表中插入7条数据，如下：
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1000", "ZhangSan", 20)
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1001", "LiSi", 21)
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1002", "WangWu", 21)
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1003", "ZhouLiu", 22)
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1004", "SunQi", 22)
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1005", "LiuBa", 23)
insert into tb_employeeinfo(employeeno, employeename, employeeage) values("A1006", "ChenShi", 25)
如果我们想要一次性从tb_employeeinfo表中查询出5条数据，该如何处理呢？
在SYBASE数据库中，可以利用“set rowcount X”语句来实现查询条数的限制。例如，本次要从tb_employeeinfo表中返回5条数据，则编写SQL语句如下：
set rowcount 5
select employeeno, employeename, employeeage from tb_employeeinfo
set rowcount 0
执行结果如下：
employeeno employeename employeeage
A1000 ZhangSan 20
A1001 LiSi 21
A1002 WangWu 21
A1003 ZhouLiu 22
A1004 SunQi 22
注意，在设置了查询条数为5并查询成功之后，一定要有“set rowcount 0”语句，否则在下次执行的时候，就最多只能返回5条数据。
例如，我们先执行如下语句：
set rowcount 5
select employeeno, employeename, employeeage from tb_employeeinfo
则此时返回的结果与上面一样。
再执行如下语句(本意是要将7条语句都查询出来)：
select employeeno, employeename, employeeage from tb_employeeinfo但此时的结果仍然只返回了5条，与我们的本意不符。
因此，在SYBASE数据库中，“set rowcount X”语句一定要与“set rowcount 0”语句配对使用。
四、总结
相比本文中的数据表，在实际的软件项目中的数据表的字段要更多一些，数据量也要更大一些，在查询语句中也有可能会带有一些其它条件，但基本的SQL语句编写模式是一致的。大家可以参照来编写对应的SQL语句。
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