var park = _context.ParkCountRecords
var area = _context.AreaInfos;
var data = from p in park
join a in area
on p.ProCode equals a.Code
group p.ParkingNum by new //ParkingNum是需要求和，求平均等的字段
{
p.ProCode, //需要进行分组依据的字段
a.ShortName //最后结果里需要取值的
} into g
select new ParkingItem
{
Name = g.Key.ShortName,
Code = g.Key.ProCode,
Num = g.Sum(), //对group的字段进行求和
};
//上述Linq to SQL语句相当于下面的sql
SELECT g.Num,g.ProCode,a.ShortName
from
(
SELECT sum(ParkingNum) as Num,ProCode
from ParkCountRecord
group by ProCode
) as g
INNER JOIN AreaInfo AS a
on g.ProCode = a.Code