去除重复的列：

mysql> select distinct deptno  from  emp;

条件查询：

mysql> select * from  emp where deptno=1 ;

mysql> select * from  emp where deptno=1 and sal < 3000;

排序和限制：

mysql> select * from emp order by sal;

mysql> select * from emp order by deptno;

mysql> select * from emp order by deptno, sal desc;

mysql> select * from emp order by sal desc limit 3;

mysql> select * from emp order by sal desc limit 1,3;

聚合：

mysql> select count(1) from emp;

mysql> select deptno,count(1) from emp group by deptno;

mysql> select deptno,count(1) from emp group by deptno with rollup;

mysql> select deptno,count(1) from emp group by deptno having count(1)>1; 

mysql> select sum(sal),max(sal),min(sal) from emp; 

表连接：

mysql> select ename,deptname from  emp,dept where emp.deptno=dept.deptno;

mysql> select ename,deptname from emp left join dept on emp.deptno=dept.deptno;

mysql> select ename,deptname from dept right join emp on dept.deptno=emp.deptno;

子查询：

mysql> select * from emp where deptno in(select deptno from dept);

mysql> select * from emp where deptno = (select deptno from dept);  

mysql> select * from emp where deptno = (select deptno from dept limit 1 );

mysql> select emp.* from emp,dept  where emp.deptno = dept.deptno ;

记录联合：

mysql> select deptno from emp union all select deptno from dept;

mysql> select deptno from emp union  select deptno from dept;