集合操作
当列的数量和列的数值类型，要完全一致，
不能确定的时候，要用null和数值类型函数来确定
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：
排序写在最后，但是来自第一个查询里面列的排序
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;
DML
insert:
SQL> create table t1(x int, y char(1), z date);
SQL> insert into t1(x, y, z) values (1, 'a', sysdate);
SQL> insert into t1(x, z, y) values (2, sysdate+1, 'b');
SQL> insert into t1(x, y, z) values (1, null, sysdate);
SQL> insert into t1(x, z) values (2, sysdate+1);
SQL> insert into t1 values (1, null, sysdate);
SQL> create table my_emp as select * from employees; （只是复制了数据和基本的表结构）
SQL> create table my_emp as select last_name, salary from employees where department_id=50;
SQL> create table avg_sal as select department_id, avg(salary) avg_sal from employees group by department_id;
SQL> create table my_emp as select * from employees where 1=0; （用不可能的条件复制表结构）
SQL> insert into my_emp select * from employees;（因为表结构相同，所以可以复制数据）
例：将department_id是50的，员工姓名和工资，保存在一张新的表里。
Create table创建空表结构，insert into插入数据
update:
SQL> update my_emp set salary=salary*1.1;
SQL> update my_emp set salary=salary*1.1 where department_id=50;
SQL> update my_emp set salary=salary*1.1, commission_pct=0.5 where employee_id=197;
delete:
SQL> delete from my_emp where employee_id=197;
SQL> delete from my_emp where department_id=50;
SQL> delete from my_emp;
子查询：
SQL> create table my_emp as select * from employees;
SQL> alter table my_emp add(department_name varchar2(30));
SQL> update my_emp outer set department_name=(select department_name from departments where department_id=outer.department_id);
update (select t1.department_name as aname,t2.department_name bname from my_emp t1 ,departments t2 where t1.department_id=t2.department_id) set aname=bname;
练习：
在new_dept表中删除没有员工的部门
SQL> create table my_dept as select * from departments;
delete from my_dept outer
where not exists
(select 1 from my_emp
where department_id=outer.department_id);
delete和truncate：
delete truncate
语句类型 dml ddl (数据定义语言)
undo数据 产生大量undo数据 不产生undo数据
空间管理 不释放 释放
语法 where 删除全部数据
（ACID : A原子性 C一致性 I 隔离性 D持久性）
dml 语句不自动提交，直到遇到commit 或者rollback才会提交
ddl 语句自动提交
dml 语句回滚(rollback)不能回滚到ddl语句前面
DML：select，insert，update，delete (CRUD）增删查改
DDL：create，alter，drop
Char 定长
Varchar2 可变长
五种约束条件：
字段：
Not null
Check(salary>0)
行与行之间：
Primary key(pk)主键约束 非空，一张表只能有一个主键
Unique 唯一性约束 同一列唯一，允许为空，一张表可以有多个唯一性约束
表与表之间：
Foreign key(references) 外键
DDL
字符串：
SQL> create table t1(x char(10), y varchar2(10));
SQL> insert into t1 values('x', 'y');
SQL> select dump(x), dump(y) from t1;
数值：
SQL> create table t1(x number(5,2), y number(5));
SQL> insert into t1 values (123.45, 12345);
SQL> insert into t1 values (12.345, 12345);
SQL> insert into t1 values (12.345, 123.45);
SQL> select * from t1;
SQL> insert into t1 values (12.345, 112345);
日期时间：
SQL> create table t1(a date, b timestamp, c timestamp with time zone, d timestamp with local time zone);
SQL> insert into t1 values (sysdate, systimestamp, systimestamp, systimestamp);
SQL> alter session set time_zone='+9:00';
SQL> select * from t1;
修改表结构：
SQL> alter table t1 add(e char(10));
SQL> alter table t1 drop(e);
SQL> alter table t1 modify(d not null);
约束条件：
字段（列）：not null, check(salary>0)
行与行：primary key, unique
表与表之间：foreign key
create table dept (
deptno int constraint dept_deptno_pk primary key,
dname varchar2(20) constraint dept_dname_nn not null);
create table emp (
empno int constraint emp_empno_pk primary key,
ename varchar2(20) constraint emp_ename_nn not null,
email varchar2(50) constraint emp_email_uq unique,
salary int constraint emp_salary_ck check(salary>0),
deptno int constraint emp_deptno_fk references dept(deptno))
SQL> select constraint_name, constraint_type from user_constraints where table_name in('DEPT', 'EMP');
SQL> insert into emp values (100, 'abc', 'abc@123.com', 10000, 10);
insert into emp values (100, 'abc', 'abc@123.com', 10000, 10)
*
ERROR at line 1:
ORA-02291: integrity constraint (HR.EMP_DEPTNO_FK) violated - parent key not
found
SQL> insert into dept values (10, 'sales');
1 row created.
SQL> insert into dept values (10, 'market');
insert into dept values (10, 'market')
*
ERROR at line 1:
ORA-00001: unique constraint (HR.DEPT_DEPTNO_PK) violated
SQL> insert into dept values (20, 'market');
1 row created.
SQL> commit;
Commit complete.
SQL> insert into emp values (101, 'def', 'def@123.com', 10000, 20);
create table emp (
empno int constraint emp_empno_pk primary key,
ename varchar2(20) constraint emp_ename_nn not null,
email varchar2(50) constraint emp_email_uq unique,
salary int constraint emp_salary_ck check(salary>0),
deptno int constraint emp_deptno_fk references dept(deptno) on delete set null)或者on delete cascade
SQl> create table t1(x int constraint t1_x_pk primary key);
SQL> alter table t1 add(y int);
SQL> alter table t1 add(z int,a int);
SQL> alter table t1 drop(z,a);
SQL> alter table t1 modify(y char(1));
SQL> alter table t1 modify(y default ‘a’);
SQL> alter table t1 disable novalidate constraint t1_x_pk;
SQL>insert into t1 values (1, ‘a’);
SQL>insert into t1 values (1, ’b’);
SQL>alter table t1 enable validate constraint t1_x_pk;报错
SQL>delete t1;
SQL>alter table t1 enable validate constraint t1_x_pk;
SQL>drop table t1;