Join Custom object with regular object

Question

I'm very new with Salesforce. I've been reading this page trying to figure out how to do joins. All the objects were created before I jumped on this project.

I have one custom object called : Person_Project_Junction__c and another one called Contact.

My query looks like this:

 SELECT
      Person_Project_Junction__c.Id,
      Person_Project_Junction__c.Person_First_Name__c,
      Person_Project_Junction__c.Person_Last_Name__c
    FROM
      Person_Project_Junction__c

I need to access a field in the Contact object called Industry__c . I tried doing this query:

  SELECT
          Person_Project_Junction__c.Id,
          Person_Project_Junction__c.Person_First_Name__c,
          Person_Project_Junction__c.Person_Last_Name__c,
          (SELECT Industry__c FROM Contact)
        FROM
          Person_Project_Junction__c

But of course, it doesnt work. How would I be able to do this join? Thanks!

Answer 1

You don't need to prefix fields with the Object's API Name.

Try this query to make sure it works fine:

list<Person_Project_Junction__c> myList = [SELECT Id,
                                                 Person_First_Name__c,
                                                 Person_Last_Name__c,
                                           FROM Person_Project_Junction__c];

Now, my question is this: Is there a lookup field to Contact on the Person_Project_Junction__c object? Assuming yes, there is, and the lookup field is called Contact__c, try this:

list<Person_Project_Junction__c> myList = [SELECT Id,Person_First_Name__c,
                                                  Person_Last_Name__c,
                                                  Contact__r.Industry__c
                                           FROM Person_Project_Junction__c];

---

Example: Let's say on the Custom Object myCustomObject__c you have a lookup field called MyLookupField__c to another sObject, say AnotherObject__c. If you want to query AnotherObject__c's field AnImportantField__c, do the following query:

list<myCustomObject__c> myCustomObjList=[SELECT id, MyLookupField__r.AnImportantField__c
                                         FROM myCustomObject__c
                                         WHERE ...];

---

Update:

Since you only need the SOQL query, here it is:

SELECT Id,Person_First_Name__c,
       Person_Last_Name__c,
       Contact__r.Industry__c
FROM Person_Project_Junction__c