1. Aggregate functions (sum, count, avg, max, min), which act on multiple records.
1、select sum(population) from country
2、select region ,sum(population) ,sum(count) from country group by region having sum(count)>1000
where filter records before aggregation
group by
Having filters group records after aggregation , and cannot use where to filter sum(count)>1000, because such a record does not exist in the country table
Query the number of employees in various positions in each department
3、select deptno,job,count(*) from emp group by deptno,job
4、select deptno,sum(sal) from emp where sal>1200 group by deptno having sum(sal)>10000 order by deptno