What kind of join to use on SQL tables

Any of those help I have four tables like this:

SELECT c.course_name, p.price_name, SUM(cp.plan_time), SUM(cp.plan_time * p.price_value)
FROM courses c
    INNER JOIN pricegroups p        ON p.price_id = c.course_price_id
    INNER JOIN course_to_plan cpl   ON cpl.course_id = c.course_id
    INNER JOIN courseplans cp       ON cp.plan_id = cpl.plan_id
GROUP BY c.course_name, p.price_name

Does that help You don't need a Join, you need to use an UNION query:

SELECT title, painterID artistID
FROM Paintings
UNION ALL
SELECT title, sculptorID artistID
FROM Sculptures

Does that help Using UNION to bring the light and medium desktops tables together as a single dataset shouldn't give you duplicates unless the tables contain duplicate values in rows accross all columns in your SELECT clauses and you use UNION ALL. If you know your tables have unique values use UNION ALL to give a performance boost.
I would combine the two tables in a common table expression (cte) and then join the resultant table with a LEFT OUTER JOIN to your user table which can also be filtered to find entries where there is no match WHERE [user].[UserID] IS NULL, NB, that would return orphaned rows in your desktops tables where the user has been deleted; alternatively drop the left outer join and use WHERE [desktop].[UserID] IS NULL to return only dekstops without assiged users.

WITH cte_Desktop AS
(
    SELECT
        [ID] as [DesktopID],
        'Light Desktop' as [DekstopType],
        [Model],
        [MACAddress],
        [UserID]
    FROM [dbo].[LightDesktops]

    UNION

    SELECT
        [ID],
        'Medium Desktop',
        [Model],
        [MACAddress],
        [UserID]
    FROM [dbo].[MediumDesktops]
)

SELECT
    [desktop].*

FROM cte_Desktop AS [desktop]
LEFT OUTER JOIN [dbo].[Users] AS [user]
    on [user].[UserID] = [desktop].[UserID]

WHERE [user].[UserID] IS NULL

I wish this help you I want to be able to join each thread at once it finished its job. In the code example below main thread will wait as long as each thread will run by their order in the list and only then next thread will be joined. , You should use a CompletableFuture

    ExecutorService e = Executors.newFixedThreadPool(3);
    ExecutorService single = Executors.newSingleThreadExecutor();
    List<CompletableFuture<?>> futures = new ArrayList<>();
    futures.add(CompletableFuture.runAsync(new Worker(), e).thenRunAsync(this::someHook, single));
    futures.add(CompletableFuture.runAsync(new Worker(), e).thenRunAsync(this::someHook, single));
    futures.add(CompletableFuture.runAsync(new Worker(), e).thenRunAsync(this::someHook, single));

    futures.forEach(f -> f.get()); // try-catch left out for brevity

This might help you You will use INNER JOINs to join your tables together on their respective/shared keys. Something like the following:

SELECT
    Students.StudentID,
    Students.Studentname,
    Courses.CourseName,
    CoursesTakes.TargetGrade,
    Reports.CurrentGrade,
    Reports.ReportNumber
FROM
    Students
    INNER JOIN CoursesTakes ON Student.StudentID = CoursesTakes.StudentId
    INNER JOIN Courses ON CoursesTakes.CourseID = Courses.CoursesID
    INNER JION Reports ON CoursesTakes.TakeID = Reports.CourseTakeID
WHERE
    Students.StudentID = <yourstudentid>