with as 用法

原语句：select * from person.StateProvince where CountryRegionCode in
(select CountryRegionCode from person.CountryRegion where Name like 'C%')

修改后：

with
cr as
(
select CountryRegionCode from person.CountryRegion where Name like 'C%'
)

select * from person.StateProvince where CountryRegionCode in (select * from cr)

with相当于一个临时表，优点是同一sql中多次调用的话不需要多次查询，感觉相当于定义了一个变量并赋值