数据库检索记录
1.连接列值：
DB2,Oracle使用||作为连接运算符。

  select ename || 'works as a' || job as msg from emp where deptno='10'

MySQL使用concat函数

  select concat(ename,'works as a',job) as msg from emp where deptno='10'  

SQL Server使用"+"号作为连接运算符

  select ename + 'works as a' + job as msg from emp where deptno='10'  

2.限制返回的行数：
DB2 ：

 select * from emp fetch first 5 rows only

MySQL ：

 select * from emp limit 5 

Oracle：

 select * from emp where rownum < 5 

3.从表中随机取出5条记录：
DB2 ：

 select * from emp order by rahnd() fetch first 5 rows only

MySQL ：

 select * from emp order by rand() limit 5 

Oracle：

 select * from ( select ename , job from emp order by dbms_random.value() ) where rownum < =5 ;