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如(ru)果在linq中希(xi)望進(jin)行一對(dui)多的復合查(cha)詢時，請直(zhi)接在查(cha)詢中使用join into，或者使用let 關鍵字(zi)，當然在建立實體(ti)時動(dong)態賦(fu)值對(dui)于查(cha)詢的性能(neng)也是沒有影響的，兩(liang)次查(cha)詢結果完成相同

一 ：在實體中賦值

var linq = from data in this.GetModel()
                       join data2 in iWebDepartmentsRepository.GetModel() on data.DepartmentID equals data2.DepartmentID
                       select new WebManageUsers_Ext
                      {
                          ManageUserID = data.ManageUserID,
                          LoginName = data.LoginName,
                          WebDepartments_Extend = data2,
                          WebManageUser_WebManageRoles_Extend = new WebManageUser_WebManageRolesRepository().GetModel().Where(i => i.ManageUserID == data.ManageUserID),
                          AvatarUrl = data.AvatarUrl,

                      };

在語句(ju)中使(shi)用let關鍵字

var linq = from data in this.GetModel()
                       join data2 in iWebDepartmentsRepository.GetModel() on data.DepartmentID equals data2.DepartmentID
                       let list = new WebManageUser_WebManageRolesRepository().GetModel().Where(i => i.ManageUserID == data.ManageUserID)
                       select new WebManageUsers_Ext
                      {
                          ManageUserID = data.ManageUserID,
                          LoginName = data.LoginName,
                          WebDepartments_Extend = data2,
                          WebManageUser_WebManageRoles_Extend = list,
                          AvatarUrl = data.AvatarUrl,

                      };

兩次的ＳＱＬ分析器的結果是完成一樣的
  

當然，對于ＬＩＮＱ的查詢，最好的方式就是“按需”查詢，也就是說，用到哪些字段就select哪些字段，代碼可能是這樣：

var linq = from data in db.WebManageUsers
                     join data2 in db.WebDepartments on data.DepartmentID equals data2.DepartmentID
                     let list1 = db.WebManageUser_WebManageRoles.Where(i => i.ManageUserID == data.ManageUserID).Select(j => new WebManageUser_WebManageRoles_Ext
                    {
                        ManageRoleID = j.ManageRoleID,
                        ManageUserID = j.ManageUserID
                    })
                     select new WebManageUsers_Ext
                     {
                         ManageUserID = data.ManageUserID,
                         LoginName = data.LoginName,
                         RealName = data.RealName,
                         WebDepartments_Extend = new WebDepartments_Ext { DepartmentName = data2.DepartmentName },
                         WebManageUser_WebManageRoles_Extend = list1,
                     };
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