Question

First – apologies for the fuzzy title, I could not find a better one.

I have table with the following structure (simplification):

EmpID DeptID

1     1
1     2
2     1
3     2
4     5
5     2

This table represents a many-to-many relationship.

I’m interested in finding all the EmpIDs that are related to a specific group of DeptIDs, for example I want all the EmpIDs that are related to DeptIDs 1, 2 and 3. Please note it’s an AND relationship and not an OR relationship. For my case, the EmpID may be related to additional DeptIDs besides 1, 2 and 3 for it to be a valid answer.

The number of DeptIDs I’m interested in changes (i.e. I may want EmpIDs who’re related to both DeptID 3 and 5, or I may want EmpIDs related to DepIDs 2, 3, 4, 5, 6, 7).

When I try to approach this problem I find myself either creating a JOIN per DepID, or a subquery per DeptID. This would mean I have to generate a new query per the number of DeptIDs I’m testing against. I would obviously prefer having a static query with a parameter or set of parameters.

I’m working over both SQL Server and MySQL (developing in parallel two versions of my code).

Any ideas?

Answer 1

I’m assuming you want to find employees that are in ALL of the specified departments and not just the employees that are in ANY of the departments, which is a far easier query.

SELECT EmpID
FROM mytable t1
JOIN mytable t2 ON t1.EmpID = t2.EmpID AND t2.DeptID = 2
JOIN mytable t3 ON t2.EmpID = t3.EmpID AND t3.DeptID = 3
WHERE DeptID = 1

I’m going to preempt the inevitable suggestion that’ll come to use aggregation:

SELECT EmpID
FROM mytable
WHERE DeptID IN (1,2,3)
GROUP BY EmpID
HAVING COUNT(1) = 3

Resist that temptation. It’s significantly slower. A similar scenario to this came up in SQL Statement – “Join” Vs “Group By and Having” and the second version was, in that second, about twenty times slower.

I’d also suggest you look at Database Development Mistakes Made by AppDevelopers.