select Sdept, count(*) from Student
group by Sdept
having Sage <= 20
select Sdept, count(*) from Student
where Sage <= 20
group by Sdept
使用where要比使用having的执行效率要高
因为where是先筛选的,因此group by语句事先分组,参与分组的数据要少,因此执行效率要高
select Sdept, count(*) from Student
group by Sdept
having Sage <= 20
select Sdept, count(*) from Student
where Sage <= 20
group by Sdept
使用where要比使用having的执行效率要高
因为where是先筛选的,因此group by语句事先分组,参与分组的数据要少,因此执行效率要高