Multi Join Linq Statement

Question

Is this the proper way to accomplish joining 3 (or more) tables with LINQ (to SQL)? Especially the select portion. Am I on the right track to return a single record(row) of data that spans across the tables?

        public static DataTable GetCurrentEmploymentQuestionnaire(Guid employmentQuestionnaireID)
    {
        var Questionnaire = from employmentQuestionnaire in context.tblEmploymentQuestionnaires
                            join prevocService in context.tblEmploymentPrevocServices on
                                employmentQuestionnaire.PrevocServicesID equals prevocService.EmploymentPrevocID
                            join jobDevelopmetService in context.tblEmploymentJobDevelopmetServices on
                                employmentQuestionnaire.JobDevelopmentServicesID equals
                                jobDevelopmetService.JobDevelopmentServicesID
                            where employmentQuestionnaire.EmploymentQuestionnaireID == employmentQuestionnaireID
                            select
                                new
                                    {
                                        EmploymentQuestionnaireID = employmentQuestionnaire.EmploymentQuestionnaireID,
                                        PlanID = employmentQuestionnaire.PlanID,
                                        HasCommunityJob = employmentQuestionnaire.CommunityJob,
                                        HasPrevocServices = employmentQuestionnaire.PrevocServices,
                                        HasJobDevelopmentServices = employmentQuestionnaire.JobDevelopmentServices,
                                        WhoCreated = employmentQuestionnaire.InsertUser,
                                        WhenCreated = employmentQuestionnaire.InsertDate,
                                        WhoUpdated = employmentQuestionnaire.UpdateUser,
                                        WhenUpdated = employmentQuestionnaire.UpdateDate,
                                        AvgRatePay = prevocService.AvgRatePay,
                                        AvgHoursWeek = prevocService.AvgHoursWeek,
                                        PrevocGoal = prevocService.PrevocGoal,
                                        SkillsTaught = prevocService.SkillsTaught,
                                        SkillsLearned = prevocService.SkillsLearned,
                                        AnticipatedTransitionPlan = prevocService.AnticipatedTransitionPlans,
                                        AnticipatedEndDate = prevocService.AnticipatedEndDate,
                                        TypeWorkDesired = jobDevelopmetService.TypeWorkDesired,
                                        NeedEmpServices = jobDevelopmetService.NeedEmploymentServices,
                                        IsDVRProvidingServices = jobDevelopmetService.DVRProvidingServices,
                                        DVRCurrentReferralExists = jobDevelopmetService.DVRCurrentReferral
                                    };

        return Questionnaire.CopyLinqToDataTable();
    }

Database structure is as follows -->

tblEmploymentQuestionnaire

EmploymentQuestionnaireID   uniqueidentifier Unchecked
PlanID                  int                     Unchecked
CommunityJob            bit                 Checked
PrevocServices          bit                 Checked
PrevocServicesID            uniqueidentifier Checked
InsertUser                  varchar(50)         Checked
InsertDate                  datetime         Checked
UpdateUser                  varchar(50)         Checked
UpdateDate                  datetime         Checked
JobDevelopmentServices  bit                 Checked
JobDevelopmentServicesID    uniqueidentifier Checked

---

tblEmploymentPrevocServices

EmploymentPrevocID  uniqueidentifier Unchecked
AvgRatePay  varchar(50) Checked
AvgHoursWeek    varchar(50) Checked
SettingID   int Checked
PrevocGoal  varchar(500) Checked
SkillsTaught    varchar(500) Checked
SkillsLearned   varchar(500) Checked
AnticipatedTransitionPlans  varchar(500) Checked
AnticipatedEndDate  datetime Checked
RatioID int Checked
rowguid uniqueidentifier Unchecked

---

tblEmploymentJobDevelopmentService

JobDevelopmentServicesID    uniqueidentifier Unchecked
TypeWorkDesired varchar(50) Checked
PreferredWorkHoursID    int Checked
NeedEmploymentServices  bit Checked
DVRProvidingServices    bit Checked
DVRCurrentReferral  bit Checked
CMOProvidingServices    bit Checked
CMONotProvidingReason   varchar(500) Checked
PaidCoachingHoursID int Checked
PlanSegregatedToIntegrated  varchar(500) Checked
RoleResponseJobDeveloper    varchar(500) Checked
RoleResponseMember  varchar(500) Checked
RoleResponseWWCTeam varchar(500) Checked
PlanDVRToWWCFund    varchar(500) Checked
DVRCurrentReferralStatusID  int Checked

---

Sorry it ended up being so long. If you are still with me, thank you and bonus appreciation points if someone can point out in the comments how I could have shortened this while still asking my question. I would appreciate it for future reference.

Answer 1

Assuming your relationships are well defined in the dbml, the generated classes should allow you to query like this:

var Questionnaire = from employmentQuestionnaire in context.tblEmploymentQuestionnaires
                    where employmentQuestionnaire.EmploymentQuestionnaireID == employmentQuestionnaireID
                    select new
                           {
                               EmploymentQuestionnaireID = employmentQuestionnaire.EmploymentQuestionnaireID,
                               PlanID = employmentQuestionnaire.PlanID,
                               HasCommunityJob = employmentQuestionnaire.CommunityJob,
                               HasPrevocServices = employmentQuestionnaire.PrevocServices,
                               HasJobDevelopmentServices = employmentQuestionnaire.JobDevelopmentServices,
                               WhoCreated = employmentQuestionnaire.InsertUser,
                               WhenCreated = employmentQuestionnaire.InsertDate,
                               WhoUpdated = employmentQuestionnaire.UpdateUser,
                               WhenUpdated = employmentQuestionnaire.UpdateDate,
                               AvgRatePay = employmentQuestionnaire.PrevocService.AvgRatePay,
                               AvgHoursWeek = employmentQuestionnaire.PrevocService.AvgHoursWeek,
                               PrevocGoal = employmentQuestionnaire.PrevocService.PrevocGoal,
                               SkillsTaught = employmentQuestionnaire.PrevocService.SkillsTaught,
                               SkillsLearned = employmentQuestionnaire.PrevocService.SkillsLearned,
                               AnticipatedTransitionPlan = employmentQuestionnaire.PrevocService.AnticipatedTransitionPlans,
                               AnticipatedEndDate = employmentQuestionnaire.PrevocService.AnticipatedEndDate,
                               TypeWorkDesired = employmentQuestionnaire.JobDevelopmentService.TypeWorkDesired,
                               NeedEmpServices = employmentQuestionnaire.JobDevelopmentService.NeedEmploymentServices,
                               IsDVRProvidingServices = employmentQuestionnaire.JobDevelopmentService.DVRProvidingServices,
                               DVRCurrentReferralExists = employmentQuestionnaire.JobDevelopmentService.DVRCurrentReferral
                            };

Basically, all the table relationships exist in your object structure. You do your query using your object relations and all the SQL joins are inferred by the attributes on your objects properties (set-up in the code generation step).

A little trick, I always place the table on the "many" side of the relationship in the from clause so I can do myObject.Parent.Property. If I have a many-to-many table it is this one that is in the from clause. Doing this, I only need to use explicit joins for:

• Left joins
• Queries that need to fetch data in a many-to-one, to-one, ... to-many. In that case the table that changes the relationship direction (from many-to-one to one-to-many) is the one in the join clause.