Linq多表合一查询

//进货
public class Buy
{
//进货表id
public int BuyID { get; set; }
//供应商id
public int SupID { get; set; }
}

//进货明细
public class BuyItems
{
//进货明细id
public int BID { get; set; }
//进货id
public int BuyID { get; set; }
//商品id
public int ProductID { get; set; }
//进货总数
public int BuyCount { get; set; }

}

//退货
public class Returns
{
//退货id
public int ReturnID { get; set; }
//供应商id
public int SupID { get; set; }
}

//退货明细
public class ReturnItems
{
//退货明细id
public int RID { get; set; }
//退货id
public int ReturnID { get; set; }
//商品id
public int ProductID { get; set; }
//退货总数
public int ReturnCount { get; set; }
}

//BuyTable为进货表(Buy)
//BuyItemsTable为进货明细表(BuyItems)
//ReturnsTable为退货表(Returns)
//ReturnItemsTable为退货明细表(ReturnItems)

var query = BuyTable.Join(BuyItemsTable, a => a.BuyID, b => b.BuyID, (a, b) => new
{
a.SupID,
b.ProductID,
BuyCount = b.BuyCount,
ReturnCount = 0,
}).Concat(ReturnsTable.Join(ReturnItemsTable, a => a.ReturnID, b => b.ReturnID, (a, b) => new
{
a.SupID,
b.ProductID,
BuyCount = 0,
ReturnCount = b.ReturnCount,
})).GroupBy(p => new { p.SupID, p.ProductID }).Select(p => new
{
p.Key.SupID,
p.Key.ProductID,
SumBuy = p.Sum(s => s.BuyCount),
SumReturn = p.Sum(s => s.ReturnCount)
}).OrderBy(p => p.SupID).ThenBy(p => p.ProductID);