疑问

--以下2语句执行结果不同

select * from dept
minus
select * from dept d where d.deptno='10'

select * from dept d where d.deptno='10'
minus
select * from dept

 

 

col  xxx for a20;--只能用于带有字符串的列，用于纯数字的列会变为#######

--查询雇佣日期为1981年的员工
select * from emp t where/* t.sal like '%1%' or*/ t.hiredate like '%81%';
select * from emp t where/* t.sal like '%1%' or*/ t.hiredate like '1981%'

--以下语句语法错误

select e.ename,
case e.sal
when e.sal < 1200 then 'e'
else 'd'
end
from emp e,salgrade s;

 

转载于:https://www.cnblogs.com/joeshang/p/10652672.html