C#两个表多条件关联写法

两个表实体类准备

 public class OtherIn
  {
      public string AfterOrderNo { get; set; }
      public long MaterialId { get; set; }
      public string MaterNumber { get; set; }
      public long Qty { get; set; }
      public string Note { get; set; }
  }

  public class AfterInfo
  {
      public string AfterOrderNo { get; set; }
      public long MaterialId { get; set; }
      public string MaterNumber { get; set; }
      public long AfterQty { get; set; }
      public string Note { get; set; }
  }

实体类数据初始化

List<OtherIn> getOtherInGroup = new List<OtherIn>();
 getOtherInGroup.Add(new OtherIn 
  { 
      AfterOrderNo= "SH231025001",
      MaterialId=100001,
      MaterNumber="CTG001",
      Qty=2
  });
  getOtherInGroup.Add(new OtherIn
  {
      AfterOrderNo = "SH231025001",
      MaterialId = 100002,
      MaterNumber = "CTG002",
      Qty = 3
  });
  getOtherInGroup.Add(new OtherIn
  {
      AfterOrderNo = "SH231025002",
      MaterialId = 100002,
      MaterNumber = "CTG002",
      Qty = 4
  });
  List<AfterInfo> getAfterGroup = new List<AfterInfo>();
  getAfterGroup.Add(new AfterInfo 
  {
      AfterOrderNo = "SH231025002",
      MaterialId = 100002,
      MaterNumber = "CTG002",
      AfterQty = 5
  });
  getAfterGroup.Add(new AfterInfo
  {
      AfterOrderNo = "SH231025001",
      MaterialId = 100001,
      MaterNumber = "CTG001",
      AfterQty = 1
  });

第一种 sql的左关联

FROM a JOIN b on 组合键关联 into 临时表 from 临时表（左关联不上右边默认为空）

  var getExcessGroup = from a in getOtherInGroup
                                 join b in getAfterGroup on new { a.AfterOrderNo, a.MaterialId } equals new { b.AfterOrderNo,b.MaterialId }
                                 into result
                                 from c in result.DefaultIfEmpty()
                                 select new
                                 {
                                     AfterOrderNo = a.AfterOrderNo,
                                     MaterialId=a.MaterialId,
                                     Qty = a.Qty,
                                     AfterQty = c == null ? 0 : c.AfterQty
                                 };

结果

第二种相当于sql的 INNER JOIN

写法一：FROM a FROM b where 多条件关联

 var getExcessGroup2 = from a in getOtherInGroup
                                  from b in getAfterGroup
                                  where a.AfterOrderNo == b.AfterOrderNo && a.MaterialId == b.MaterialId
                                  select new
                                  {
                                      AfterOrderNo = a.AfterOrderNo,
                                      MaterialId = a.MaterialId,
                                      Qty = a.Qty,
                                      AfterQty = b == null ? 0 : b.AfterQty
                                  };

结果

写法二： FROM JOIN INTO

FROM a JOIN b on 组合键关联 into 临时表 from 临时表（不默认为空）

var getExcessGroup1 = from a in getOtherInGroup
                                 join b in getAfterGroup on new { a.AfterOrderNo, a.MaterialId } equals new { b.AfterOrderNo, b.MaterialId }
                                 into result
                                 from c in result
                                 select new
                                 {
                                     AfterOrderNo = a.AfterOrderNo,
                                     MaterialId = a.MaterialId,
                                     Qty = a.Qty,
                                     AfterQty = c == null ? 0 : c.AfterQty
                                 };

结果

写法三： FROM JOIN 省略into

var getExcessGroup11 = from a in getOtherInGroup
                                  join b in getAfterGroup on new { a.AfterOrderNo, a.MaterialId } equals new { b.AfterOrderNo, b.MaterialId }
                                  select new
                                  {
                                      AfterOrderNo = a.AfterOrderNo,
                                      MaterialId = a.MaterialId,
                                      Qty = a.Qty,
                                      AfterQty = b.AfterQty
                                  };

结果