基础的增删改查，数据库优化，索引

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create database test1;
drop database test1;
use test1;
create table emp(ename varchar(10),hiredate date,sal decimal(10,2),deptno int(2));
drop table emp;
alter table emp modify ename varchar(20);
alter table emp add column age int(3);
alter table emp drop column age;
alter table emp change age age1 int(4);
alter table emp add birth date after ename;
alter table emp modify age int(3) first;
alter table emp rename emp1;

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insert into emp(ename,hiredate,sal,deptno) values('zzx1','2000-10-11',2000,1);
insert into emp values('lisa','2004-05-09',3000,2);
insert into dept values(5,'dept5'),(6,'dept6');
update emp set sal=4000 where ename='lisa';
update emp a,dept b set a.sal=a.sal*b.deptno,b.deptname=a.ename where a.deptno=b.deptno;
delete from emp where ename='dony';
delete a,b from emp a,dept b where a.deptno=b.deptno and a.deptno=3;
select * from emp where ename='lisa';
select distinct deptno from emp;
select * from emp order by sal(desc);
select * from emp order by sal limit 5;
select * from emp order by sal limit 1,5;ss

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create index cityname on city(city(10));
drop index cityname on city;