SQL Nested Inner Joins

Question

Im trying to use the results of an inner join in another query and having trouble getting my head around how.

This is the first query i am running:

SELECT projects.Project_ID, projects.Name
FROM projects
INNER JOIN assigned_users
ON assigned_users.Project_ID=projects.Project_ID AND assigned_users.User_ID=4;

This is getting all of the assigned projects for a particular user. The Project_ID's this query returns i want to use to find all the related requirements for those projects.

SELECT *
FROM requirements
WHERE requirements.Project_ID=1;

So instead of finding the requirements for project '1' i want to get the requirements for all projects assigned to a particular user. Any help would be appreciated.

Cheers

Answer 1

If I understand correctly, you would just add another JOIN :

SELECT . . .
FROM assigned_users au JOIN
     projects p
     ON au.Project_ID = p.Project_ID JOIN
     requirements r
     ON r.Project_ID = p.Project_ID
WHERE au.User_ID = 4;

Answer 2

You can use either:

SELECT *
FROM requirements req
INNER JOIN projects pro 
    ON req.Project_ID = pro.Project_ID
INNER JOIN assigned_users u
    ON u.Project_ID=pro.Project_ID
WHERE u.User_ID=4;

or

SELECT *
FROM requirements
WHERE requirements.Project_ID=(
    SELECT Project_ID
    FROM projects
    INNER JOIN assigned_users
    ON assigned_users.Project_ID=projects.Project_ID AND assigned_users.User_ID=4;
);

Answer 3

To the existing query, we can just add:

  JOIN requirements
    ON requirements.Project_ID = projects.Project_ID

We add expressions to the SELECT list to retrieve values of columns from requirements.

SELECT projects.Project_ID, projects.Name
     , requirements.some_column
     , requirements.some_other_column

It's likely that we also want to add an ORDER BY clause so that the rows are returned in a predictable sequence.

Note that if there are no matching rows in requirements for a given Project_ID , then that project will not be returned. To return those rows, we can specify an outer join.

Current query:

SELECT projects.Project_ID
     , projects.Name
  FROM projects
  JOIN assigned_users
    ON assigned_users.Project_ID=projects.Project_ID
   AND assigned_users.User_ID=4

Proposed query with outer join to requirements table:

SELECT projects.Project_ID
     , projects.Name
     , requirements.Requirement_ID
     , requirements.Requirement_Name 
  FROM projects
  JOIN assigned_users
    ON assigned_users.Project_ID=projects.Project_ID
   AND assigned_users.User_ID=4
  LEFT
  JOIN requirements 
    ON requirements.Project_ID = projects.Project_ID
 ORDER
    BY projects.Project_ID
     , requirements.Requirement_ID

Equivalently, we could relocate the condition assigned_users.User_ID=4 from the ON clause of the inner join to a WHERE clause, before the ORDER BY .