EF 里的 join and Group Join

join

var skip = pageSize * (pageIndx - );
pageCount = _db.Orders.Count();
return _db.Orders.OrderByDescending(c=>c.ID).Skip(skip).Take(pageSize)
 .Join(_db.Income, a => a.ID, b => b.OrderId, (a, b) => new OrdersInComeModel {
 ID=a.ID,
 incometype=b.incometype,
 Amount=a.Amount,
 BuyDate=a.BuyDate,
 CancelDate=a.CancelDate,
 IncomeAmount=b.IncomeAmount,
 IncomeDate=b.IncomeDate,
 State=a.State
 }).ToList();

left join

var skip = pageSize * (pageIndx - );
pageCount = _db.Orders.Count();
var lsw= _db.Orders.OrderByDescending(a => a.ID).Skip(skip).Take(pageSize);
var rt=(from l in lsw join i in _db.Income on l.ID equals i.OrderId into os
from f in os.DefaultIfEmpty()
select new OrdersInComeModel
{
ID = l.ID,
incometype = f.incometype,
Amount = l.Amount,
BuyDate = l.BuyDate,
CancelDate = l.CancelDate,
IncomeAmount = f.IncomeAmount,
IncomeDate = f.IncomeDate,
State = l.State
}).ToList();

Group Join

一个KEY对象对应一个集合（返回子表对应集合）

var data = dbContext.Categories.GroupJoin(dbContext.Products, c => c.CategoryID, p => p.CategoryID, (c, p) => new { CategoryID = c.CategoryID, ProductList = p });