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select deptno avy(sal) from emp group by deptno;（每个部门的平均工资）

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select deptno,count(ename) from emp;

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select deptno,job sum(sal)

from emp

group by deptno,job

order by 1;

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（1）select deptno,job sum(sal) from emp group by deptno,job;

（2）select deptno sum(sal) from emp group by deptno;

（3）select sum(sal) from emp;

（4）select deptno,job sum(sal) from emp group by rollup( deptno,job);

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group by rollup(a,b)

==

group by a,b

+

group by a

+

没有group by