Conditional WHERE clause using a mapping table - sql

Question

I have two tables like below:
1.MAPPING -
ID Mapping ID
A01 P01
A03 P03
2.MAIN -
ID Metric1 Metric2
A01 (null) 4.22
A02 (null) (null)
A03 145 127
A04 (null) 20
P01 68 (null)
P03 (null) (null)
I have to run query for a particular ID. But I have to check two conditions :
If that ID exists in MAPPING table and if one of the Metrics is null
then I have to take Metric value of the Mapping ID.
Suppose for A01 , it exists in MAPPING and Metric1 is null so we have to take the value of Metric1 of P01 i.e. 68. But for Metric2 as it is not null so
we don't have to consider P01.
Now suppose for A02 , it is not in the table MAPPING . So we will
show as it is.
I am using Oracle. Can you please help me how to make an efficient query for this ?

Answer 1

This?
SQL> with
2 mapping (id, mapping_id) as
3 (select 'A01', 'P01' from dual union all
4 select 'A03', 'P03' from dual
5 ),
6 main (id, metric1, metric2) as
7 (select 'A01', null, 4.22 from dual union all
8 select 'A02', null, null from dual union all
9 select 'A03', 145, 127 from dual union all
10 select 'A04', null, 20 from dual union all
11 select 'P01', 68, null from dual union all
12 select 'P03', null, null from dual
13 )
14 select n.id,
15 case when n.metric1 is null then x.metric1 else n.metric1 end metric1,
16 case when n.metric2 is null then x.metric2 else n.metric2 end metric2
17 from main n left join mapping g on n.id = g.id
18 left join main x on x.id = g.mapping_id
19 order by n.id;
ID METRIC1 METRIC2
--- ---------- ----------
A01 68 4,22
A02
A03 145 127
A04 20
P01 68
P03
6 rows selected.
SQL>

Answer 2

Consider query that uses Main twice, joined to each field of Mapping:
SELECT Main.ID, IIf([Main].[Metric1] Is Null,[Main_1].[Metric1],Main.Metric1) AS M1,
IIf([Main].[Metric2] Is Null,[Main_1].[Metric2],Main.Metric2) AS M2
FROM Main AS Main_1
RIGHT JOIN (Main LEFT JOIN Mapping ON Main.ID = Mapping.ID)
ON Main_1.ID = Mapping.MappingID;