小编Ash*_*mc4的帖子

我有这张桌子

create table student (
   stu_id int,
   s_name nvarchar(max),
   s_subject nvarchar(max),
)

这是数据

insert into student values(123,'pammy','English');
insert into student values(123,'pammy','Maths');
insert into student values(123,'pammy','Chemistry');
insert into student values(124,'watts','Biology');
insert into student values(125,'Tom','Physics');
insert into student values(125,'Tom','Computer';
insert into student values(125,'Tom','ED';

所以我想检索已发生两次以上的记录.我的代码是

select stu_id,s_Name 
from student 
group by stu_id,s_Name 
having count(stu_id) >2 ;

结果很完美.

但是当我想要s_subject它时也没有选择任何行.我不知道为什么.

select stu_id,s_Name,s_subject 
from student 
group by stu_id,s_Name,s_subject 
having count(stu_id) >2 ;

我的桌子是 -

create table mobile
(
  id integer,
  m_name  varchar(20),
  cost integer
)

而价值观是 -

insert into mobile values(10,'NOkia',100);
insert into mobile values(11,'Samsung',150);
insert into mobile values(12,'Sony',120);

我知道如何计算柱成本的平均值,我的代码是 -

 select avg(cost) from mobile;

结果是123

但我想计算平均值,然后也显示差异.我能够这样但但是,我无法在选择查询中添加avg列 -

我的代码是---

SELECT id, m_name as "Mobile Name", cost as Price,avg(cost) as Average,
cost-(select avg(cost) from mobile) as Difference FROM mobile
group by id,m_name,cost;

输出是 -

id      Mobile Name   Price  Average  Difference 
10      Nokia         100    100      -23
11      Samsung       150    150       27
12      Sony …

我有账户表 -

create table account
(
acct_id int,
cust_id int,
cust_name varchar(20)  
)

insert into account values(1,20,'Mark');
insert into account values(2,23,'Tom');
insert into account values(3,24,'Jim');

我想创建一个触发器,它将确保不能在帐户表中插入或更新记录,其中acct_id为2,cust_id为23.

我的代码是 -

create trigger tri_account
before insert or update
on account
for each row
begin
    IF (:new.acct_id == 2 and :new.cust_id == 23) THEN 
        DBMS_OUTPUT.PUT_LINE('No insertion with id 2 and 23.');
rollback;
    END IF; 
end;

所以这个触发器是创建的,但是有编译错误.现在,当我插入任何acct_id为2且cust_id为23的记录时,它会执行allow.但是我收到错误说 ORA-04098:触发'OPS $ 0924769.TRI_ACCOUNT'无效且重新验证失败

我不明白这一点.我也希望显示一条消息,说明不能插入.请帮忙...