group by

--使用group by語句查詢

select count(studName) from student_1 group by study

select count(uname) from userluo group by age;

根據(jù)某一個(gè)字段進(jìn)行分組

select age,count(*) from userluo group by age;

select uname,count(*) from userluo group by uname;

矛盾。。。。。

盡管deptno和deptname是一一對(duì)應(yīng)的，但group by不知道。。。按照語法來檢查。。。

Group by 后面也可以有多個(gè)字段，但是要求和前面一致。。。。。。。。

-- 單列分組

--顯示每個(gè)部門的平均工資和最高工資

Select deptno,avg(sal),max(sal) from emp

group by deptno;

-- 多列分組

--顯示每個(gè)部門、每種崗位的平均工資和最高工資

Select deptno,job,avg(sal),max(sal) from emp

group by deptno,job; -- 注意： deptno 和 job 完全相同才顯示一條記錄。

Having 和 group by 配套使用。。。

可以在 group 分組以后再過濾，

select age,count(*) from userluo group by age having count(*)<2;

--使用having語句

select trim(studName),count(*) from student_1 group by trim(studName) having count(*)=2;

select count(*) studName from student_1 group by studName having count(*)=2;

order by

--使用order by語句,desc是降序排列，asc是升序排列,默認(rèn)為升序排列

select * from student_1 order by age desc;

select * from student_1 order by age sname desc;先按age排列，相同的age再按sname排列，

group by ，order by ，having