Question

I am trying to write a query using the following example tables:

Company

• CompanyID
• Name
• County

CompanyRelation

• ParentCompanyID
• ChildCompanyID

CompanySpecialty

• CompanyID
• Specialty

The CompanyRelation table is used because each company can have many subcompanies under it and each subcompany can have many companies over it.

Ideally, what I want to be able to find is all companies by a certain county and/or by a certain specialty. Now, normally, I would just join Company to CompanySpecialty and filter on those two fields, but here is where it gets tricky: the subcompanies don’t have counties assigned to them, so if I filter on the county, all the subcompanies will be excluded.

If I filter the companies down to a county, I would like to display all subcompanies that are associated to that company, regardless of the fact that the subcompany does not have an address.

To get around this, I have to bring in the CompanyRelation table.

Example data:

CompanyID---Name---County
1-----------ABC----King
2-----------BCD----Pierce
3-----------DEF----NULL
4-----------EFG----NULL

ParentCompanyID---ChildCompanyID
1-----------------1
1-----------------3
2-----------------2
2-----------------4

CompanyID---Specialty
1-----------Vehicles
2-----------Vehicles
3-----------Vehicles
4-----------Vehicles

Using this data, say I want to find all companies in King county that deal with vehicles. In my results, I would expect to see Company 1 and Company 3.

How can I write a query to accomplish this?

Answer 1

The Diagram would be like this (with Company being the primary / parent table, and all others depending on it):

Company Specialty <– Company <– Relationship <– Company –> Company Specialty

Converting it to SQL would be like this:

SELECT
   --whatever fields you want, as long as you prefix with the table alias (p., ps., dr., c., cs.)
FROM
      Company Parent P --parent

   Left Join CompanySpecialty PS
   ON P.CompanyID = PS.CompanyID

   Left Join CompanyRelation CR
   ON P.CompanyID = CR.ParentCompanyID

   Left Join Company C
   ON CR.ChildCompanyID = C.ComapnyID

   Left Join Company Specialty CS
   ON C.CompanyID = CS.CompanyID
WHERE
   (PS.Specialty = 'Vehicle' AND P.County = 'King')
   OR
   (CS.Specialty = 'Vehicle' AND C.County = 'King')