Conditional operations without using SWITCH CASE - sql

Question

I have a couple of complex tables. But their mapping is something like:
TABLE_A:
_________________________________
| LINK_ID | TYPE_ID |
_________________________________
| adfasdnf23454 | TYPE 1 |
| 43fsdfsdcxsh7 | TYPE 1 |
| dfkng037sdfbd | TYPE 1 |
| sd09734fdfhsf | TYPE 2 |
| khsadf94u5dfc | TYPE 2 |
| piukvih66wfa8 | TYPE 3 |
_________________________________
TABLE_B:
_____________________________________________
| LINK_ID | CODE_ID | CODE_VALUE |
_____________________________________________
| adfasdnf23454 | 7 | test 1 |
| fgubk65esdfj7 | 6 | test 2 |
| ooogfsg354fds | 7 | test 3 |
| sd09734fdfhsf | 5 | test 4 |
_____________________________________________
The LINK_ID column links these two tables.
My requirement is to have all the records from TABLE_A checked whether they have a specific CODE_ID or not.
If the record has CODE_ID as 7 - populate CODE_VALUE in a column.
If the record has CODE_ID as 6 - populate CODE_VALUE in another column.
If the record doesn't have a CODE_ID show CODE_VALUE as null.
The catch is, TABLE_B may have records that TABLE_A don't. But the final result should contain the records of TABLE_A alone.
PS: SWITCH CASE not suggested since I would require the fields in the same row. (Please see the multiple rows for same LINK_ID in OBTAINED RESULT on using SWITCH CASE)
OBTAINED RESULT:
_______________________________________________
| LINK_ID | CODE_VALUE_1 | CODE_VALUE_1 |
_______________________________________________
| adfasdnf23454 | test 1 | null |
| adfasdnf23454 | null | test 4 |
| sd09734fdfhsf | test 6 | null |
_______________________________________________
EXPECTED RESULT:
__________________________________________________
| LINK_ID | CODE_VALUE_1 | CODE_VALUE_2 |
__________________________________________________
| adfasdnf23454 | test 1 | test 4 |
| 43fsdfsdcxsh7 | null | null |
| dfkng037sdfbd | null | null |
| sd09734fdfhsf | test 6 | null |
| khsadf94u5dfc | null | null |
| piukvih66wfa8 | null | null |
__________________________________________________
Can someone help on this?

Answer 1

One option uses two correlated subqueries:
select
a.link_id,
(select code_value from table_b b where b.link_id = a.link_id and b.code_id = 7) code_value_1,
(select code_value from table_b b where b.link_id = a.link_id and b.code_id = 6) code_value_2
from table_a a
Note that this assumes no duplicate (link_id, code_id) in table_b. You could also write this with two left joins - which is quite the same logic.
Another solution is a single left join, then conditional aggregation:
select
a.link_id,
max(case when b.code_id = 7 then b.code_value end) code_value_1,
max(case when b.code_id = 6 then b.code_value end) code_value_2
from table_a a
left join table_b b on b.link_id = a.link_id and b.code_id in (6, 7)
group by a.link_id

Answer 2

Problematic part of your question is what to do if two entries in B have same link_id and type_id. You can use min, max, last entry (but for that you need ordering column in B). Or you can list them all:
select *
from a left join b using (link_id)
pivot (listagg(code_value, ', ') within group (order by code_value)
for code_id in (6 as code6, 7 as code7))
Data:
create table a (link_id, type_id) as (
select 'a', 'TYPE 1' from dual union all
select 'b', 'TYPE 1' from dual union all
select 'c', 'TYPE 1' from dual union all
select 'd', 'TYPE 2' from dual );
create table b(LINK_ID, CODE_ID, CODE_VALUE) as (
select 'a', 6, 'test 1' from dual union all
select 'a', 7, 'test 2' from dual union all
select 'a', 7, 'test 3' from dual union all
select 'b', 7, 'test 4' from dual union all
select 'd', 6, 'test 5' from dual );
Result:
LINK_ID TYPE_ID CODE6 CODE7
a TYPE 1 test 1 test 2, test 3
b TYPE 1 test 4
c TYPE 1
d TYPE 2 test 5
dbfiddle