创建触发器的语法：
Create trigger 触发器的名字
after insert/update/delete/(select是没有触发器的)
on 表名字
declare begin dbms_output.put_line("使用触发器打印一句话"); end;
/
语法明细：
CREATE [or REPLACE] TRIGGER 触发器的名字
{BEFORE | AFTER}
{DELETE| UPDATE | INSERT}
ON 表名
{FOR EACH ROW (WHERE条件)}
PL/SQL块
触发器创建完毕，在插入数据的时候出现如下错误：
在行: 12 上开始执行命令时出错 -
insert into emp(empno,ename,sal,deptno) values(1001,'Jack',50000,10)
错误位于命令行: 12 列: 13
错误报告 -
SQL 错误: ORA-04098: 触发器 'SCOTT.SAYNEWEMP' 无效且未通过重新验证
04098. 00000 - "trigger '%s.%s' is invalid and failed re-validation"
*Cause: A trigger was attempted to be retrieved for execution and was
found to be invalid. This also means that compilation/authorization
failed for the trigger.
*Action: Options are to resolve the compilation/authorization errors,
disable the trigger, or drop the trigger.