How to select rows with related values? - sql

Question

I have a table "company" with data like follows containing employees and their companies:
EmployeeID EmployeeName CompanyName CompanyID ParentCoID
------------------------------------------------------------------------------------------------
100 John A 500 NULL
100 John A.1 600 500
250 Paul B 800 NULL
250 Paul ABC 2000 NULL
350 Joe D 1000 5000
600 Tom E 700 NULL
600 Tom E.2 1500 700
I am trying to display rows where if the Parent Co ID value is in the Company ID, then show both rows for that employee. So basically select employees that are in both the parent company and the sub company and show both of those rows.
I've tried
SELECT *
FROM company
WHERE CompanyID IN (SELECT ParentCoID FROM company)
but this only returns the row where the CompanyID value matches and I'm not sure how to also include that second row also.
My desired output for the sample above would be:
EmployeeID EmployeeName CompanyName CompanyID ParentCoID
------------------------------------------------------------------------------------------------
100 John A 500 NULL
100 John A.1 600 500
600 Tom E 700 NULL
600 Tom E.2 1500 700
As from the result above, Company A.1 is a sub company of A, and same with company E and E.2. I am trying to select employees that are in both the main company and sub company and therefore need to refer to the ParentCoID and the CompanyID columns.

Answer 1

SELECT * FROM company
WHERE EmployeeID IN (SELECT EmployeeID FROM company WHERE CompanyID IN (SELECT ParentCoID FROM company)

Answer 2

Please try this:
Updated the query.
SELECT * FROM company c
WHERE CompanyID IN (SELECT ParentCoID FROM company c2 WHERE c2.EmployeeID = c.EmployeeID) OR ParentCoID IN (SELECT CompanyID FROM company c3 WHERE c3.EmployeeID = c.EmployeeID)

Answer 3

Using EXISTS
SELECT a.*
FROM company a
WHERE EXISTS (SELECT b.parentcoid
FROM company b
WHERE a.companyid = b.parentcoid
AND a.employeeid = b.employeeid)
UNION ALL
SELECT c.*
FROM company c
WHERE EXISTS (SELECT d.companyid
FROM company d
WHERE d.companyid = c.parentcoid
AND d.employeeid = c.employeeid)

Answer 4

A recursive CTE is useful in querying hierarchical data.
with cte As (
select * from company c
Where ParentCoID is null and CompanyID = Any(Select ParentCoID From company Where EmployeeID=c.EmployeeID)
union all
select c.*
from cte p inner join company c On (p.CompanyID=c.ParentCoID And p.EmployeeID=c.EmployeeID)
)
select * From cte order by EmployeeID, CompanyID, ParentCoID

Answer 5

Here's a slightly different approach using a simple window function and a self-join and will likely be performant given a clustered index on (EmployeeId,ParentCoId).
with e as (
select EmployeeID,ParentCoID CoId,
Sum(case when ParentCoID is null then 1 end ) over(partition by EmployeeID) IsSub
from Company
)
select c.*
from e
join Company c on c.EmployeeID=e.EmployeeID
where (e.CoId=c.CompanyID or e.CoId=c.ParentCoID)
and e.CoId is not null and e.IsSub=1