Ignore clauses in where based on if you get a result from the select - sql

Question

I want the following sql statement to give me different results based on whether it finds a result within the specific string or it needs to search the whole parent group (so within the table there are for example:
column 1, column 2
a - 1
a - 2
a - 3
b - 5
b - 7
b - 1
so if it can find the result if i put 1 it will display me
a -1
b -1
. the problem is that in the where clause exist both the parent group and the child group
i have tried to use case and also to simulate an if with ands and ors but it didn't work
select 1,
aapv.aapv_keyext1,
aapv.aapv_area,
aapv.aapv_valuecharmax,
aapv.aapv_valuechardefault,
aapv.aapv_valuecharmin, aap.aap_ident
from a_parameter_value aapv,
a_parameter aap
where aap.aap_ident in (string1,string2,string3)
and aap.aap_ref = aapv.aap_ref
and aap.aap_idento = string4
and ((aapv.Aapv_Keyext1 = 'LaD1' --child clause
and aapv.aapv_keyext1 is not null)
or aapv.Aapv_Area = 'LSDe' --parent clause
and aapv.Aapv_Area is null)
I expect the output to be if the aapv_keyext1 value finds any results then the appv_area is not used at all but either only the child clause is used with the above code or both if i remove the is null clause

Answer 1

Okay, you need to provide more information for us to give you a real answer, but I wanted to point out that this section has some logic problems:
and ((aapv.Aapv_Keyext1 = 'LaD1' --child clause
and aapv.aapv_keyext1 is not null)
or aapv.Aapv_Area = 'LSDe' --parent clause
and aapv.Aapv_Area is null)
The first part is saying aapv_keyext1 = 'LaD1' AND aapv_keyext1 is not null; the second half can never be false, so it's redundant. The second part is saying aapv_area = 'LSDe' AND aapv_area is null. This will never be true. So this whole section is equivalent to:
and (aapv.aapv_keyext1 = 'LaD1')
Which probably isn't what you want. You say you want "if the aapv_keyext1 value finds any results then the appv_area is not used at all". I suspect what you mean is that "if any results exist for aapv_keyext1 in any rows then don't use aapv_area" which is more complicated, you need a subquery (or analytic/aggregate functions) to look at what other rows are doing.
select 1,
aapv.aapv_keyext1,
aapv.aapv_area,
aapv.aapv_valuecharmax,
aapv.aapv_valuechardefault,
aapv.aapv_valuecharmin, aap.aap_ident
from a_parameter_value aapv,
a_parameter aap
where aap.aap_ident in (string1,string2,string3)
and aap.aap_ref = aapv.aap_ref
and aap.aap_idento = string4
and (-- prefer keyext1
aapv.Aapv_Keyext1 = 'LaD1'
OR
-- if keyext1 doesn't find results...
(NOT EXISTS (select 1 from a_parameter_value aapv2
where aapv2.aap_ident = aap.aap_ident
and aap2.aap_ref = aap.aap_ref
and aap2.aap_idento = aap.aap_idento
and aapv.Aapv_Keyext1 = 'LaD1')
AND
-- ... use aapv_area
aapv.Aapv_Area = 'LSDe')
);
You can also do this kind of conditional logic with CASE statements, but you're still going to need a subquery or something if you want your logic to depend on the values in rows other than the one currently being looked at.
Let me know if I've misunderstood your question and I'll try to update with a better answer.