集合操作

desc job_history：改变历史职位

job_history：历史表

vnion：重复值只保留一个 去除重复值 ★★

vnion all: 把所有重复值保留 不去除重复值★★

intersect：相交

minus：减去

双引号对不规范的对象命名★★

单引号是一个字符串★★

desc departments 查询部门表里面的有哪些列

desc employees 查询员工表里面的有哪些列

select employee_id, job_id from employees
union all
select employee_id, job_id from job_history;

select employee_id, job_id from employees
union
select employee_id, job_id from job_history;

select employee_id, job_id from employees
intersect
select employee_id, job_id from job_history;

select employee_id from employees
minus
select employee_id from job_history;

select employee_id, job_id, salary from employees
union all
select employee_id, job_id, null from job_history;

select employee_id, job_id, to_char(salary) from employees
union all
select employee_id, job_id, ‘no salary‘ from job_history;

集合排序：
select employee_id, job_id, salary from employees
union all
select employee_id, job_id, null from job_history
order by salary;

select employee_id, job_id, null from job_history
union all
select employee_id, job_id, salary from employees
order by 3;

salary null跟上空值 个人理解

创建表：

create table：创建表

insert into：插入

update：更新

delete：删除表里的数据

drop：删除表

constraint：约束条件 commit：提交 创建表完事后 一定要提交 commit；

not null：非空

view：视图 drop sequence course_cid; 删除增量的名字 如果重复就用这个删除 coure_cid为你创建的要删除的重复名字

sequence：序列

uniquenes：唯一性 start with:从....开始

varchar2：长字符

modify：修改，添加

主键(primary key)约束、外键(foreign key)约束、唯一(unique)约束、检查(check)约束、默认值(default)约束实例

约束的类型有如下几种：
C (check constraint on a table)
P (primary key)
U (unique key)
R (Referential AKA Foreign Key)
V (with check option, on a view)
O (with read only, on a view)

DML

x：第一列+数据类型

y：第二列+字符类型——字符类型必须加单引号

z：第三列+日期类型

select * from + 表名+想查的东西

desc+表名 也可以查

删除表：drop table + 表名

user_table：当前用户下所有表的名字

删除行（删除数据）：delete from table + 哪一行

DDL

修改表结构

alter table t1 xxxxxx ★★

删除列：alter t1 drop x

create table t1（x（列名）int（类型） constraint（约束条件） t1_x_pk(约束的名字）primarykey（主键））；

Oracle课程档案，第五天