Listing data that share same data - sql

Question

I have three tables:
SN | Table Name | Primary Key | Foreign Key | Attribute
-----------------------------------------------------------------
1 | salesArea | salesAreaID | | areaDescription
2 | store | storeID | salesAreaID |
3 | salesPerson | salesPersonID | storeID | salesPersonName
I am trying to get the salesPersonName(s) of anyone that is in the same salesArea as a certain salesPerson.
For instance, a salesPerson named David works in storeID 23, which is in the NE salesArea.
There is another salesPerson named Bob working in storeID 34, which is also in the NE salesArea.
My query so far is...
SELECT salesPersonName,
salesPersonID,
st.salesAreaID,
areaDescription,
sp.storeId
FROM salesperson as sp, salesArea as sa, store as st
I am confused on how to get it get the storeID of 'David' and then retrieve what the salesArea is. Then with that salesArea, retrieve all salesPersons with that salesArea.
Salesperson table only gives the storeID, but then store gives the salesArea for the storeIDs.
Point in the right direction would be nice, join clause? some fancy where with a group by?

Answer 1

I am assuming:
Areas have stores
stores have salespersons
salespersons have names
So, we first want to find the area ID of the salesperson passed:
SELECT sp.SalesPersonID, st.SalesAreaID
FROM salesperson as sp
INNER JOIN store as st on sp.StoreID = st.StoreID
WHERE sp.SalesPersonName = 'David'
Now, we could use the results of this query to then execute the second part of the question, which is "get the other people in a given salesarea". If, for example, the above query returned SalesAreaID = 23 and SalesPersonID = 10, our second query would look like the following:
SELECT sp.SalesPersonID, sp.SalesPersonName, st.StoreID
FROM stores st
INNER JOIN salespersons sp on st.StoreID = sp.StoreID
WHERE st.SalesAreaID = 23 and sp.SalesPersonID <> 10
But, we can combine the two queries to form a single one that does the same thing:
SELECT sp2.SalesPersonID, sp2.SalesPersonName, st2.StoreID
FROM salesperson as sp
-- Join to stores to get the areaID of the salesperson passed (multiplicity 1)
INNER JOIN store as st on sp.StoreID = st.StoreID
-- self join to store to get all stores in that area (multiplicity 1 x Stores[area])
INNER JOIN store as st2 on st.SalesAreaID = st2.SalesAreaID
-- get all salespersons assigned to all stores in the area (multiplicity 1 x salespersons[area])
INNER JOIN salesperson as sp2
on st2.StoreID = sp2.StoreID
-- only return "other" salespersons
and sp2.SalesPersonID <> sp.SalesPersonID
WHERE sp.SalesPersonName = 'David'
Et voilà, we have a query giving all other salespersons in a the same area as a passed salesperson.