Oracle join using two possible column data - sql

Question

I work with an Oracle 12 database that represents mainframe data. Here is my question.
We have two levels of heirarchy, "System" and "Prin". Imagine them as state and county in the USA. Sometimes, a client will build everything at System level and all of it's children will always referece the System configuration. Other clients built at Prin level, and and child of the prin will first have to look at the PRIN level data for configuration, if prin is not built in the table, then it defaults to the system level config. Pretty easy.
Here's where i can't get the table join to work. A single client can have some systems built at the systems level, and others at the prin level. How can i dynamically join when i am not sure what configuration the client is using in that specific prin?
Example:
WITH tbl as (
select 80 SYSTEM, 0 PRIN, 2 DATA from dual
union
select 80 , 1 , 3 from dual
union
select 80 , 2 , 4 from dual
)
now if i have an item located in system 80 prin 3... it will need the 0 prin data because 0 denotes the "system" config.
so if i have prin 1, i want data "3". if i have prin 2, data "4" , if i have prin 8, i want data "2" because there is no prin 8 config built.
See where I am trying to get?
So when i do
select *
from tbl t
inner join tbl2 tt on t.sys = tt.sys and prin = ?????
how do I say "if prin is built in tbl, use prin, otherwise default to prin = 0"
I know this is a badly stated question. So please ask more specifics and i will try to answer quickly. This is affecting multiple tables.

Answer 1

Pretty ugly, but then so is the data model...
with
tbl ( s, prim, val ) as (
select 80, 0, 2 from dual union all
select 80, 1, 3 from dual union all
select 80, 2, 4 from dual
),
inputs ( s, prim ) as (
select 80, 1 from dual union all
select 80, 5 from dual
)
select t.s, i.prim i_prim, t.prim tbl_prim, t.val
from tbl t join inputs i
on t.s = i.s
and
( t.prim = i.prim
or t.prim = 0
and not exists ( select * from tbl
where s = i.s and prim = i.prim ))
;
S I_PRIM TBL_PRIM VAL
---- ---------- ---------- ----------
80 5 0 2
80 1 1 3
2 rows selected.

Answer 2

I would disadvise you from using a (complex) JOIN with OR-condition on slightly bigger tables (50k+), as the execution plan may totally go nuts from my own experience.
Under such circumstances rather use a Union (select cond1_match) union all (select cond2_default) ordered/ranked and select the first row or use a JOIN like
select coalesce(a1.prin, a2.prin)
from (select cond1_match) a1
full join (select cond2_default) a2
And if understand you right, that you have just one number as input and want to join another datatable, then my suggestion would look like this
with
tbl ( SYSTEM, PRIN, DATA ) as (
select 80, 0, 2 from dual union all
select 80, 1, 3 from dual union all
select 80, 2, 4 from dual
),
tbl2 (SYSTEM, PRIN, OTHERDATA) as (
select 80, 0, 99 from dual union all
select 80, 1, 333 from dual union all
select 80, 2, 444 from dual
)
select t.system, t.prin, t.data, tt.otherdata
from tbl t
inner join tbl2 tt on t.system = tt.system and t.prin = tt.prin
where t.prin = (select nvl(max(prin), 0) from tbl where system = t.system and prin = :pri)
;
system + prin have to be unique or max() would be random
:pri = 5
SYSTEM PRIN DATA OTHERDATA
------ ---- ---- ---------
80 0 2 99
:pri = 2
SYSTEM PRIN DATA OTHERDATA
------ ---- ---- ---------
80 2 4 444
Only guessing about tbl2 and the join condition, but that's basically how I was told to look up data or use a default if no_data_found in SQL