oracle 性能优化操作十二： 用Case语句合并多重扫描

select count(*) from emp where sal<1000;

select count(*) from emp where sal between 1000 and 5000;

select count(*) from emp where sal>5000;

select

count (sale when sal <1000

then 1 else null end)              count_poor,

count (sale when between 1000 and 5000

then 1 else null end)              count_blue_collar,

count (sale when sal >5000

then 1 else null end)              count_poor

from emp;