6.2 运算符
6.2.1 算术运算符
加 / 减 / 乘 / 除
6.2.2 连接运算符
是用来连接字符串的。跟java中的 + 是一致的。
select 'abc' || ' bcd ' as 连接后的结果 from dual;
select d.dname || ' 部门' from dept d;
6.2.3 比较运算符
> / < / >= / <= / != / <> /IS NULL / Like / Between / In / Exsist
-- 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;
6.2.4 逻辑运算符
and / or / not
not 的优先级 > and 的优先级 > or 的优先级
6.2.5 集合操作符
Union / UnionAll / Intersect / Minus
-- 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;