grabbing information by joining multiple tables - sql

Question

This is going to be a little complicated. Let me start with my tables.
clients [src = 0]
---------
clientID code company
--------- ------- ---------
1 ABC ABC Corp
2 DEF DEF Corp
carriers [src = 1]
---------
clientID code company
--------- ------- -------
1 ABC ABC Inc.
2 JHI JHI Inc.
link
--------
contactID uID src
--------- ----- ----
1 1 0
1 1 1
1 2 0
contact info
--------------
contactID fname lname
--------- ------- --------
1 John Smith
2 Quincy Jones
So, i'm trying to do a search for say "ABC" on the link table. The link table needs to basically join to either the carriers or clients table depending on the link.src column. It should find two matches, one in the clients and one in the carriers, but since both resolve to contactID (links table) of 1, i should then query the contact info table and return
Found 1 record(s):
John Smith
I hope this makes sense. Any help is greatly appreciated!

Answer 1

Here is one approach using left join:
select co.*
from link l left join
clients cl
on l.src = 0 and l.uid = cl.code left join
carriers ca
on l.src = 1 and l.uid = ca.code left join
contacts co
on l.contactid = co.contactid
where 'ABC' in (co.code, cl.code)

Answer 2

Here is another approach. First, you UNION the Clients and Carriers tables and add a new column ContactType to differentiate one from the other. Use 0 for Clients and 1 for Carriers, the same as src. Then you perform a LEFT JOIN to get the desired result.
;WITH Clients(ClientID, Code, Company) AS(
SELECT 1, 'ABC', 'ABC Corp' UNION ALL
SELECT 2, 'DEF', 'DEF Corp'
)
,Carriers(ClientID, Code, Company) AS(
SELECT 1, 'ABC', 'ABC Inc.' UNION ALL
SELECT 2, 'JHI', 'JHI Inc.'
)
,Link(ContactId, UID, Src) AS(
SELECT 1, 1, 0 UNION ALL
SELECT 1, 1, 1 UNION ALL
SELECT 1, 2, 0
)
,ContactInfo(ContactID, FName, LName) AS(
SELECT 1, 'John', 'Smith' UNION ALL
SELECT 2, 'Quincy', 'Jones'
)
-- START
,Contact(ContactID, ContactType, Code, Company) AS(
SELECT
ClientID, 0, Code, Company
FROM Clients
UNION ALL
SELECT
ClientID, 1, Code, Company
FROM Carriers
)
SELECT DISTINCT
ci.FName,
ci.LName
FROM Link l
LEFT JOIN Contact c
ON c.ContactID = l.UID
AND c.ContactType = l.src
LEFT JOIN ContactInfo ci
ON ci.ContactID = c.ContactID
WHERE
c.Code = 'ABC'

Answer 3

Look at this from a modeling pov. You have two tables with the same type of data in each one, the entity company. The only difference between them is their role or relationship to your company. So why not keep them all in the same bucket?
Companies:
ID code Name
-- ---- ---------
1 ABC ABC Corp
2 DEF DEF Corp
3 JHI JHI Inc.
If a particular company could only be a client or a carrier, that designation could be placed in the Companies table. Since obviously one company can be both, the designation goes into a separate table. The following shows that company 1, 'ABC', is both a client ('L') and carrier ('R'), company 2 is only a client and company 3 is only a carrier.
CompanyRoles:
CompanyID Type
--------- ----
1 'L'
1 'R'
2 'L'
3 'R'
There is no need to keep multiple copies of the same data just because a company can play multiple roles. If there is role-dependent data, data that is maintained on client but not for carriers, or vice versa, then subtables can keep that.
As for the contacts, if a company has one contact no matter the role, the contact reference can be added to the Companies table. If the contact is role dependent, it is added to the CompanyRoles table.
CompanyRoles:
CompanyID Type ContactID
--------- ---- ---------
1 'L' 1
1 'R' 2
2 'L' 3
3 'R' 4
Want to see a list of clients?
select c.ID as ClientID, c.Code as ClientCode, c.Name as ClientName,
ci.ContactName
from Companies c
join CompanyRoles cr
on cr.CompanyID = c.ID
and cr.Type = 'L'
left join Contacts ct -- In case no contact is currently defined
on ct.ContactID = cr.ContactID
join ClientSpecificData csd
on csd.ClientID = c.ID;
Want to see a list of carriers?
select c.ID as CarrierID, c.Code as CarrierCode, c.Name as CarrierName,
ci.ContactName
from Companies c
join CompanyRoles cr
on cr.CompanyID = c.ID
and cr.Type = 'R'
left join Contacts ct -- In case no contact is currently defined
on ct.ContactID = cr.ContactID
join CarrierSpecificData csd
on csd.ClientID = c.ID;
You can create views on the last two queries to provide a single data source for those apps that deal with only Clients or only Carriers. Triggers on the views can deal with incoming DML statements as needed to route the data to the appropriate tables.
As you can see, the queries are clean and simple. Data integrity is easy and scalability is not an issue. What more could you want?