SQL Query to get missing staff member - sql

Question

I have following problem to find out if a User with a specific job role is missing in one project:
Table 1:
ID | Project_ID
---+------------
1 | 11A
1 | 11B
1 | 11C
2 | 12B
2 | 12C
3 | 13A
3 | 13C
Table 2:
Project_ID | JobRole_ID
-------------+------------
11A | A
11B | B
11C | C
12B | B
12C | C
13A | A
13C | C
Table 3:
JobRole_ID | JobRole
-----------+---------
A | Manager
B | Project Leader
C | Project Assistent
For each project jobrole A,B and C are required (Table 3). Table 2 only contains added JobRoles, not missing ones.
What i expect is:
ID | JobRole
---+---------
1 | Manager
2 | NULL
3 | Manager
Please help me! Thx

Answer 1

You want to find Projects where exists a JobRole which is not tied to this particular project.
From top of my head:
--try to find projects
SELECT
T1.Id
FROM
Table1 AS T1
WHERE
--with at least one role
EXISTS(
SELECT
*
FROM
Table3 AS T3
WHERE
--where mapping does not exist
NOT EXISTS(
SELECT
*
FROM
Table2 AS T2
WHERE
T2.Project_ID = T1.Project_ID AND
T2.JobRole_ID = T3.JobRole_ID
)
)
EDIT:
Could this be what you want? If not, could you please give us more details?
SELECT
T1.Id, T3.JobRole_ID
FROM
Table1 AS T1 LEFT JOIN
Table2 AS T2 ON T2.Project_ID = T1.Project_ID LEFT JOIN
Table3 AS T3 ON T3.JobRole_ID = T2.JobRole_ID

Answer 2

Based on your updated requirements this should work, although I'm sure it can be simplified.
select a.ID, b.JobRole
from (
select * from Table1, table3 where JobRole_ID = 'A'
) a left join (
select t1.id, t3.jobrole
from table1 t1
left join table2 t2 on t2.project_id = t1.project_id
left join table3 t3 on t3.jobrole_id = t2.jobrole_id
) b on a.id = b.id and a.jobrole = b.jobrole
group by a.ID, b.JobRole

Answer 3

Here is a method:
select p.*, j.*
from projects p cross join
jobroles j left join
projectjobs pj
on p.project_id = pj.project_id and
j.jobrole_id = pj.jobrole_id
where pj.project_id is null;
It generates a list of all projects and all jobs (using the cross join). Then, using the left join and where clause, it filters out the ones that exist.
EDIT:
You can try with comma:
select p.*, j.*
from projects p,
jobroles j left join
projectjobs pj
on p.project_id = pj.project_id and
j.jobrole_id = pj.jobrole_id
where pj.project_id is null;
But this may not work because of the semantics of how the comma is parsed in the from clause. You might need a subquery:
select xpj.*
from (select p.*, j.*
from projects p, jobroles j
) xpj left join
projectjobs pj
on xpj.project_id = pj.project_id and
xpj.jobrole_id = pj.jobrole_id
where pj.project_id is null;