Question

I have two tables; A and B. Table A contains a column of GUIDs. Table B contains a column of GUIDs and a column of ClientIDs. In table B, the ClientIDs may be matched to multiple GUIDs. If given 1 GUID, how would I select all GUIDs from table A that belong to the same ClientID as the given GUID?

For Example, if Table A has three rows:

Row1 | 11111111-2222-3333-4444-555555555555  
Row2 | 22222222-3333-4444-5555-666666666666  
Row3 | 33333333-4444-5555-6666-777777777777

Table B has 3 rows:

Row1 | ClientA | 11111111-2222-3333-4444-555555555555  
Row2 | ClientB | 22222222-3333-4444-5555-666666666666  
Row3 | ClientA | 33333333-4444-5555-6666-777777777777  

I am given a GUID (11111111-2222-3333-4444-555555555555), how would I write a select statement to find all GUIDs from A that match the ClientID from Table B? (11111111-2222-3333-4444-555555555555 & 33333333-4444-5555-6666-777777777777)

Answer 1

Eh, how about first finding the client with the given guid from table b

SELECT client FROM table_b WHERE guid = <given_guid>

and then getting all guid’s for that client from table b

SELECT guid FROM table_b WHERE client = <client>

I mean you could do it in one go using subqueries

SELECT guid FROM table_b WHERE client IN ( 
    SELECT client FROM table_b WHERE guid = <given_guid>
)

but why complicate things

If you need additional data from table a for the guid’s for clients, you can either JOIN or use the above as another subquery, e.g.

SELECT guid FROM table_a WHERE guid IN (
    SELECT guid FROM table_b WHERE client IN ( 
        SELECT client FROM table_b WHERE guid = <given_guid>
    )
)

p.s. I hate using subqueries, there almost always is a better way