select 'abc' || ' bcd ' as 连接后的结果 from dual;

select d.dname || ' 部门' from dept d;

-- 判断 null 值

select * from n5 where s is null;

select * from n5 where s is not null;

-- like 模糊查询。慎用，有可能会导致全表扫描，效率低。

-- % 匹配0到多个字符，_ 匹配一个字符

select username from dba_users where username like 'VI_';

select username from dba_users where username like 'SC%';

select username from dba_users where username like '%SC%';

-- in，是 where x = a or x = b or x = c 的一种缩写。下面两条是等价的。

select * from emp where empno in (700, 800, 900);

select * from emp where empno = 700 or empno = 800 or empno = 900;

-- in 后面跟的不一定是逗号分隔的单项，也可能是一个完整的查询语句。

-- 下面两条结果是一致的

-- 这种 in 慎用

select * from emp where deptno in (select deptno from dept where dname = 'SALES');

select a.* from emp a,dept d where a.deptno =d.deptno and d.dname='SALES';

-- between...and

select * from emp where empno between 7800 and 9000;

-- 等同于：

select * from emp where empno >= 7800 and empno <= 9000;

-- 生成测试数据

create table dept_01 as select * from dept where rownum < 6;

create table dept_02 as select * from dept where rownum < 4;

insert into dept_02 values (98, '小吃部', '斗门');

insert into dept_02 values (99, '外卖部', '香洲');

commit;

select * from dept_01

union

select * from dept_02;

select * from dept_01

union all

select * from dept_02;

select * from dept_01

intersect

select * from dept_02;

select * from dept_01

minus

select * from dpet_02;