nvl(p1,p2): 如果p1为空,就转换成p2;
SQL> select ename,sal + nvl(comm,0) "工资" from emp;
nvl2(p1,p2,p3):如是p1不为null,返回p2,如果为null,返回p3;
SQL> select ename, nvl2(comm,sal+comm,sal) "工资" from emp;
case
when p1 then q1
[when p2 then q2
else q3]
end
select ename,hiredate,
case
when to_char(hiredate,'yyyy-mm-dd')<'1981-01-01' then '老一辈'
when to_char(hiredate,'yyyy-mm-dd')<'1985-01-01' then '中层干部'
else '新人'
end
from emp
分组函数:avg, count,max ,min,sum
avg: select avg(nvl2(comm,comm,0)) from emp;
注意：
１．在select 列表中的字段必须是组函数(avg,count,max,min,sum) 中的参数或者group by 子句．
２．在group by p1 ,p1可以不出现在查询列表中.
３．where语句中不允许出现组函数(avg,count,max,min,sum) ,用having代替．
select deptno,max(sal) from emp group by deptno having max(sal)>2000;
语句执行顺序，where->group by->having->order by
