I have never used JOIN. When is proper time? - asp.net-mvc

Question

I have never used JOIN(INNER, OUTER), and I don't have any idea when is the best scenario:
Here I have two examples of ActionResults that uses 2 or 3 queries to get an object, isn't better to use a JOIN instead?
First Example:
public ActionResult JobTags(int id)
{
var jobTagsList = (from j in db.JobTags
where j.JobID == id
select j.TagID).ToList();
var tags = (from j in db.Tags
where jobTagsList.Contains(j.ID.ToString())
select j).ToList();
return View(tags);
}
Can I just JOIN this two tables and select that j at the end?
Second Example:
public ActionResult ImageListWhoApp(int id)
{
//We get here the ID from Job page using dbo.Jobs
var userIdList = (from j in db.Jobs
where j.ID == id
select j.ID.ToString()).ToList();
//We get here who applied at this job using dbo.AppliedJobs
var appJobIdList = (from j in db.AppliedJobs
where userIdList.Contains(j.JobID.ToString())
select j.UserID).ToList();
//Finally we get here the avatars of the user who applied at the job
//We are using this as a hyperlink to user profile.
var appUserImage = (from j in db.Images
where appJobIdList.Contains(j.UserID.ToString())
select j).ToList();
return View(appUserImage);
}
Isn't this approach getting ridiculous? Or is normal to do stuff like this in this manner? How do I make a JOIN from this 3 SQL statements? Is it possible? is it the better way?
Thank you for your time!

Answer 1

You don't need joins. You can use navigation properties:
var tagsQry =
from tag in db.Tags
where tag.JobTag.JobID == id
select tag;
var userImageQry =
from img in db.Images
from appJob in db.AppliedJobs
where (img.UserID == appJob.UserID) && (appJob.Job.ID == id)
select img;
Even if you have no navigation properties, you don't need joins:
var tagsQry =
from tag in db.Tags
from jobTag in sb.JobTags
where (jobTag.JobID == id) && (tag.ID == jobTag.TagID)
select tag;
var userImageQry =
from img in db.Images
from appJob in db.AppliedJobs
from job in db.Jobs
where (img.UserID == appJob.UserID) && (appJob.JobID == job.ID) && (job.ID == id)
select img;
You can however use joins if you prefer the syntax. The DB side query execution plan will be exactly the same:
var tagsQry =
from tag in db.Tags
join jobTag in sb.JobTags on tag.ID equals jobTag.TagID
where (jobTag.JobID == id)
select tag;
var userImageQry =
from appJob in db.AppliedJobs
join img in db.Images on appJob.UserID equals img.UserID
join job in db.Jobs on appJob.JobID equals job.ID
where (job.ID == id)
select img;
In the second example you only need the query (or join) to Jobs if you don't have a foreign key constraint on AppliedJobs.JobID. If you do, you can compare AppliedJobs.JobID directly with id.