06:where语句：select * from emp where deptno = 10;
select * from emp where ename = 'clark';
select ename,sal from emp where sal > 1500;
select ename,sal,deptno from emp where deptno <> 10;//<>:不等于
select ename,sal from emp where ename > 'CBA';//比较ascii码大小；
select ename,sal from emp where sal between 800 and 1500;
空值处理：select ename,sal,comm from emp where comm is (not) null;
in语句：select ename,sal,comm from emp where sal in <800,1500,2000>
select ename,sal,comm from emp where ename in ('SMITH','KING','ABC')
日期处理：select ename,sal,hiredate from emp where hiredate > '20-2月-81';
过滤条件连接：where deptno = 10 and sal > 10;where deptno = 10 or sal > 10;where not in (800,1500);
模糊查询：where ename like '%ALL%';
where ename like '_A%';//%代表一个或多个字符，下划线代表一个；
where ename like '%\%%';//'\'是转义字符
where ename like '%$%%' escape '$';//表明￥是转移字符
07:数据的排序order by：直接select，按插入顺序显示。
select * from dept order by deptno desc;//desc:descent,降序排列
select empno,ename from emp order by empno asc;//asc:ascent,升序（默认）
select empno,ename from emp where deptno <> 10 order by deptno asc;//先过滤后排序
select ename,sal,deptno from emp order by deptno asc,ename desc;//先按deptno升序。后按ename降序
08:常见的sql语句：lower语句：select lower(ename) from emp;select ename from emp where lower(ename) like '_a%';
uper语句；
substring：select substr(ename,2,3) from emp;
char(把一个数字转换成一个字符):select chr<65> from dual;
ascii:select ascii<'A'> from dual;
round:select round<23.652,2> from dual;//四舍五入；
to_char:select to_char from emp;//重点，必须记住！
注：L代表本地符号；若9改为0，前面加0；
select hiredate from emp;
select to_char(hiredate,'YYYY-MM-DD HH:MI:SS') FROM EMP;
select to_char(sysdate,'YYYY-MM-DD HH24:MI:SS') FROM dual;
select to_char(sysdate,'YYYY-MM-DD HH:MI:SS') FROM dual; //对日期的转换必须记住！
09:to_date函数：select ename,hiredate from emp where hiredate > '1981-2-20 12:34:54';
select ename,hiredate from emp where hiredate > to_date ('1981-2-20 12:34:54','YYYY-MM-DD HH24:MI:SS');
to_number函数：select sal from emp where sal>to_number<'$1,250.00','$9,999.99'
nvl函数（空值处理）：select ename sal*12+NVL from emp;
10:select max(sal)/min(sal)/avg(sal) from emp;
select to_char(avg(sal),'99999999.99') from emp;//数字转换成字符串
select round(avg(sal),2) from emp;//四舍五入到小数点后面两位；
select sum(sal) from emp;//支出薪水
select count(*) from emp;//求出这个表总共有多少条记录
select count(*) from emp where deptno = 10;//count<*>:count所有字段
组函数：max，min，avg，sum，count//重点！必记！
select count(ename) from emp;//count(ename):这个字段如果不是空值就算一个
select count(distinct deptno) from emp;//去除重复值以后还有多少个部门编号