Get record for a key but display value for a separate key in DB2 SQL

Question

So I have a table which has a composite primary key for SHIP# & REF#. Each SHIP# has two codes for REF# which are BM and PO. BM field is mandatory but the PO field only gets populated when the user actually inputs something. So, a basic select would display something like this:

SHIP#           REF#    VALUE
000002          BM      20001836                      
000002          PO      020                           
000003          BM      20001834                      
000003          PO      8-694                         
000004          BM      20001835                      

Now, you will notice that shipment 000004 has only BM and no PO.

I want to display all shipments with PO's values. So, if the PO value is empty or doesn't exist like in case '000004', it should simply put '-'. Since the BM is mandatory you'll have to get all records where BM exists but display value of the PO field.

So, the output should be:

SHIP#           REF#    VALUE                     
000002          PO      020                           
000003          PO      8-694                         
000004          PO      -  

Let me know if you need more clarifications. Thanks.

Answer 1

You can use aggregation:

select ship#, 'PO' as ref#,
       max(case when ref# = 'PO' then value end) as value 
from t
group by ship#

This returns the value as NULL -- which seems like a very good choice. If you really want '-' , then use COALESCE() :

select ship#, 'PO' as ref#,
       coalesce(max(case when ref# = 'PO' then value end), '-') as value 
from t
group by ship#

Answer 2

An outer join against itself can do the job too. For example:

select a.ship, 'PO' as ref, coalesce(b.value, '-') as value
from t a
left join t b on b.ship = a.ship and b.ref = 'PO'
where a.ref = 'BM'

Result:

SHIP    REF  VALUE
------  ---  -----
000002  PO   020
000003  PO   8-694
000004  PO   -

See running example at db<>fiddle .

EDIT - Find only BMs with no PO.

You can use the same query and add the extra predicate and b.ship is null in it, as in:

select a.ship, 'PO' as ref, coalesce(b.value, '-') as value
from t a
left join t b on b.ship = a.ship and b.ref = 'PO'
where a.ref = 'BM' 
  and b.ship is null

Result:

SHIP    REF  VALUE 
------- ---- ----- 
000004  PO   -     

See running example at db<>fiddle .