select ProID,ProName from T_Province

select CityID,CityName from T_City
where ProID = (
    select ProID from T_Province where ProName="湖南省"
);

select ProID,ProName from T_Province
union
select CityID,CityName from T_City
where ProID = (
    select ProID from T_Province where ProName="湖南省"
);

select ProID,ProName from T_Province
union all
select CityID,CityName from T_City
where ProID = (
    select ProID from T_Province where ProName="湖南省"
);

select count(CityID) from T_City
where ProID = (select ProID from T_Province where ProName="湖北省")

select ProName,CityName from（
    T_City join T_Province
    on T_City.ProID = T_Province.ProID
）
where ProName="湖北省"

select T_City.ProID,ProName,count(CityID) as cc from(
    T_City join T_Province
    on T_City.ProID = T_Province.ProID
)
group by T_City.ProID
order by cc desc;

select tc.ProID,ProName,count(CityID) as cc from(
    T_City tc join T_Province tp
    on T_City.ProID = T_Province.ProID
)
group by tc.ProID
order by cc desc;

select CityName,count(DisName) disCount from (
    T_City tc join T_District td
    on tc.CityID = td.CityID
)
group by CityName
having disCount > 20;

select tp.ProName,tcd.CityName,tcd.ci from
(
select ProID,CityName,count(ID) ci from(T_City tc join T_District td on tc.CityID = td.CityID) 
    
group by tc.CityID
order by ci desc
limit 3
)tcd
join T_Province tp on tcd.ProID = tp.ProID;

select * from(
T_Province tp join T_City tc
on tp.ProID = tc.ProID
);

select * from(
T_Province tp left join T_City tc
on tp.ProID = tc.ProID
);

select * from(
T_Province tp right join T_City tc
on tp.ProID = tc.ProID
);