Filtering data based on dynamic date - oracle-sqldeveloper

Question

Create table #TEMP
(
ID INT
)
Create table #TEMP1
(
ID INT,
Letter_Type VARCHAR(100),
Letter_Sent_Date DATE
)
INSERT INTO #TEMP VALUES (1),(2),(3),(4)
GO
INSERT INTO #TEMP1 VALUES
(1,'A','01/01/2017'),
(1,'B','01/02/2017'),
(1,'C','01/03/2018'),
(1,'D','01/04/2018'),
(2,'A','01/01/2017'),
(2,'B','01/02/2017'),
(2,'C','01/10/2018'),
(2,'D','01/12/2018')
I'm trying to achieve below results - data should be based on date.
Suppose I want to know any letter sent after '01/05/2018' for letter type C.
For ID 1 there is no letter C - in that case, we need to print null value.
I'm trying to do it in single statement as query I currently have is super big due to couple of joins used.
Any help is much appreciated. Thanks!
OUTPUT
1,NULL,NULL
2,C,'01/10/2018'

Answer 1

This is rather clumsy, but I think that it returns what you want (written in Oracle).
CTE (WITH factoring clause) is used to prepare test data; you don't need that, as your data already is in those two tables). Useful code begins at line 15; have a look at the comments.
SQL> with
2 temp (id) as
3 (select level from dual connect by level <= 4),
4 temp1 (id, letter_type, letter_sent_date) as
5 (select 1, 'A', date '2017-01-01' from dual union all
6 select 1, 'B', date '2017-01-02' from dual union all
7 select 1, 'C', date '2018-01-03' from dual union all
8 select 1, 'D', date '2018-01-04' from dual union all
9 --
10 select 2, 'A', date '2017-01-01' from dual union all
11 select 2, 'B', date '2017-01-02' from dual union all
12 select 2, 'C', date '2018-01-10' from dual union all
13 select 2, 'D', date '2018-01-12' from dual
14 ),
15 -- letter type = 'C' and letters were sent after 2018-01-05
16 satisfy_condition as
17 (select t.id, t1.letter_type, t1.letter_sent_date
18 from temp t join temp1 t1 on t1.id = t.id
19 where t1.letter_type = 'C'
20 and t1.letter_sent_date > date '2018-01-05'
21 )
22 -- Finally: rows that satisfy condition ...
23 select i.id, i.letter_type, i.letter_sent_date
24 from satisfy_condition i
25 union
26 -- ... and rows that contain letter type = 'C', but were sent before 2018-01-05
27 select t.id, null, null
28 from temp1 t
29 where t.letter_type = 'C'
30 and t.id not in (select i1.id from satisfy_condition i1)
31 order by id, letter_type, letter_sent_date;
ID LETTER_TYPE LETTER_SENT_DATE
---------- ------------ --------------------
1
2 C 2018-01-10
SQL>

Answer 2

In an effort to avoid not in /not exist solution maybe this can help you?
WITH TEMP (ID, LETTER_TYPE, LETTER_SENT_DATE) AS
(select 1, 'A', date '2017-01-01' from dual union all
SELECT 1, 'B', DATE '2017-02-01' FROM DUAL UNION ALL
--SELECT 1, 'C', DATE '2018-03-01' FROM DUAL UNION ALL
select 1, 'D', date '2018-04-01' from dual union all
select 2, 'A', date '2017-01-01' from dual union all
SELECT 2, 'B', DATE '2017-02-01' FROM DUAL UNION ALL
SELECT 2, 'C', DATE '2018-10-01' FROM DUAL UNION ALL
select 2, 'D', date '2018-12-01' from dual
)
SELECT ID, LETTER_TYPE, LETTER_SENT_DATE FROM TEMP
WHERE LETTER_SENT_DATE > DATE '2018-05-01'
AND LETTER_TYPE = 'C'
UNION
SELECT ID, NULL LETTER_TYPE, NULL LETTER_SENT_DATE
from (
select tt.id , sum(tt.HASLETTER) hasletter
FROM
(SELECT T1.ID, CASE WHEN (T1.LETTER_TYPE='C') THEN 1 ELSE 0 END HASLETTER --, NULL T1.LETTER_SENT_DATE
FROM TEMP T1) TT
GROUP BY TT.ID
) hl
where hl.hasletter = 0