省市区3表查询
根据选择的省，来查询出市区

create table province(
id int identity(1,1) primary key,
pname varchar(20)
)
create table citys(
cid int identity(1,1) primary key,
cname varchar(20),
cpid int constraint cp_id references province(id) 
)
create table district(
did int identity(1,1) primary key,
dname varchar(20),
cdid int constraint cd_id references citys(cid) 
)

[解决办法]
市查询
select * from citys where pid = 1
区查询
select * from district where cdid in (
select cid from citys where pid = 1
)