null： select * from emp where comm is null;
nvl函数：select sal, comm,sal+nvl(comm,0) as 工资 from emp;
派生列：select empno，ename，'地址不祥' as 地址 from emp;
字符串相加：select ename,sal,ename||sal from emp;
查询日期：select sysdate from dual;
授权：grant select,update,insert on emp to dang;
解权：revoke all on emp from dang;
查询结构：select desc emp;
删除重复项：select distinct job from emp;
修改表名和派生列：select t.* , sal+comm from emp t;
范围查询：
select * from emp where empno in（10,20）;
select * from emp where empno not in（10,20）;
模糊查询，通配符：_：一个字符；%：任意长度字符
select job from emp where job like '_N';
insert into emp value （211231，'tom','make%',null,null,null,null,null）;
转义字符，可以将特殊字符作为普通字符处理，查询job中带有/的用户信息：select * from emp where job like '%/%' escape '/';
插入日期：insert into emp
---to_date把字符转转换成日期类型需要两个参数，一是被转的字符串，二是要转换成的格式.
把日期转换为字符串：
select ename,to_char(hiredate,'yyyy"年"mm"月"dd"日"') from emp;
select * from emp where hiredate<to_char('1981-1-1','yyyy-mm-dd');
取余：select mod （sal，2） from emp;
给系统日期加3个月：select add_months(sysdate,3) from dual;
全部变大写、小写，首字母大写：select upper('wewer'),lower('enter'),initcapz('he she') from dual;
ename列全部变小写：select lower(ename) from emp;
字符串拼接：selecte concat('hello','world') from dual;
截取第二个开始往后两个字符：select substr('234567890',2,2) from dual;
@在哪个位置：select instr('aswo@','wo@') from dual;
往左补18个字符：select lpad('sd',20,'*') from dual
往右补18个字符：select rpad('sd',20,'*') from dual
把指定字符串从左右两边去掉：select trim('h' from 'hhhherhhhwwehhh') from dual;结果：erhhhww
替换：select replace('18396868677','8','*') from dual;
小数部分去掉,：select mod(trunc(12.6),4) from dual;
数据类型转换：select to_char(45.232,'$999.99') from dual;$45.23
select to_number('324') from dual;324
char2[20]:可变长度的数据类型
plan_type为1则变为*否则为空：select plan_name,decode(plan_type,'1','*') as 领导交办 from t;
select plan_name,decode(plan_type,'2','*') as 临时任务 from t;
--创建一查询，显示与Blake在同一部门工作的雇员的姓名和受雇日期，--但是Blake不包含在内。select ename,hiredate from emp where deptno in (select deptno from emp where ename='BLAKE') and ename <> 'BLAKE';
--显示位置在Dallas的部门内的雇员姓名、编号以及工作
SELECT * FROM EMP WHERE DEPTNO IN (SELECT DEPTNO FROM DEPT WHERE LOC='DALLAS');
--显示被King直接管理的雇员的姓名以及工资。
SELECT A.* FROM EMP A,EMP B WHERE A.MGR=B.EMPNO AND B.ENAME='KING';
select * from emp where mgr=(select empno from emp where ename='KING');
--创建一查询，显示能获得与Scott一样工资和奖金的其他雇员的姓名、受雇日期以及工资
SELECT * FROM EMP WHERE (SAL,NVL(COMM,0))=(SELECT SAL,NVL(COMM,0) FROM EMP WHERE ENAME='SCOTT');