Union two select statements while keeping distinct for not null column - sql

Question

My Oracle statement has two parts:
Select statement 1 is returning rows as:
a b c NULL
a x y NULL
Select statement 2 is returning rows as:
a b c d
e f g h
I want to union both the selects provided for a row having same columns(except NULL column) as in select 2 , only the not NULL row is returned.
Output:
a b c d
a x y NULL
e f g h
CHANGED REQUIREMENTS:
The requirements are bit changed now and i have case like:
Select statement 1 as:
a b c e NULL
a x y s NULL
Select statement 2 as:
a b c d text
e f g h text
Output:
a b c d text
a x y s NULL
e f g h text
I.e. in case of NULL field in last column, I need to fetch the row from "Select statement 2".

Answer 1

Considering that first three columns are not nullable you can use FULL OUTER JOIN:
with t1 as (
select 'a' c1, 'b' c2, 'c' c3, null c4 from dual
union all
select 'a', 'x', 'y', null from dual),
t2 as (
select 'a' c1, 'b' c2, 'c' c3, 'd' c4 from dual
union all
select 'e', 'f', 'g', 'h' from dual)
select c1, c2, c3, coalesce(t1.c4, t2.c4) c4
from t1 full outer join t2 using(c1, c2, c3);
C1 C2 C3 C4
-- -- -- --
a b c d
e f g h
a x y (NULL)
According to updated requirements:
with t1(c1, c2, c3, c4, c5) as (
select 'a', 'b', 'c', 'e', null from dual
union all
select 'a', 'x', 'y', 's', null from dual),
t2(c1, c2, c3, c4, c5) as (
select 'a', 'b', 'c', 'd', 'qwerty' from dual
union all
select 'e', 'f', 'g', 'h', 'asdfgh' from dual)
select c1,
c2,
c3,
nvl(nvl2(t1.c5, t1.c4, t2.c4), t1.c4) c4,
coalesce(t1.c5, t2.c5) c5
from t1
full outer join t2
using (c1, c2, c3);
C1 C2 C3 C4 C5
-- -- -- -- ------
a b c d qwerty
e f g h asdfgh
a x y s (NULL)
This is fast and dirty hack. Although it works on sample data you provided, it might behave unpredictable on your full dataset. This just gives you an idea how to accomplish your goal. I strongly recommend you to test it thoroughly before use.

Answer 2

Suppose that we got TABLE1 and TABLE2, in this query TAB1 returns same rows in tables and remove the null columns from that rows and TAB2 returns all rows from TABLE1 with out the rows which are the same with the rows in TABLE2 and TAB3 is same TAB2 but the tables changed, and finally UNION ALL TAB1 and TAB2 and TAB3:
SELECT * FROM
(SELECT
CASE WHEN T1.COL1 IS NULL THEN T2.COL1 ELSE T1.COL1 END COL1,
CASE WHEN T1.COL2 IS NULL THEN T2.COL2 ELSE T1.COL2 END COL2,
CASE WHEN T1.COL3 IS NULL THEN T2.COL3 ELSE T1.COL3 END COL3,
CASE WHEN T1.COL4 IS NULL THEN T2.COL4 ELSE T1.COL4 END COL4
FROM
TABLE2 T2 INNER JOIN TABLE1 T1 ON
(T1.COL1 = T2.COL1 OR T1.COL1 IS NULL OR T2.COL1 IS NULL) AND
(T1.COL2 = T2.COL2 OR T1.COL2 IS NULL OR T2.COL2 IS NULL) AND
(T1.COL3 = T2.COL3 OR T1.COL3 IS NULL OR T2.COL3 IS NULL) AND
(T1.COL4 = T2.COL4 OR T1.COL4 IS NULL OR T2.COL4 IS NULL))TAB1
UNION ALL
SELECT * FROM
(SELECT * FROM TABLE1
MINUS
SELECT T1.*
FROM
TABLE2 T2 INNER JOIN TABLE1 T1 ON
(T1.COL1 = T2.COL1 OR T1.COL1 IS NULL OR T2.COL1 IS NULL) AND
(T1.COL2 = T2.COL2 OR T1.COL2 IS NULL OR T2.COL2 IS NULL) AND
(T1.COL3 = T2.COL3 OR T1.COL3 IS NULL OR T2.COL3 IS NULL) AND
(T1.COL4 = T2.COL4 OR T1.COL4 IS NULL OR T2.COL4 IS NULL))TAB2
UNION ALL
SELECT * FROM
(SELECT * FROM TABLE2
MINUS
SELECT T2.*
FROM
TABLE2 T2 INNER JOIN TABLE1 T1 ON
(T1.COL1 = T2.COL1 OR T1.COL1 IS NULL OR T2.COL1 IS NULL) AND
(T1.COL2 = T2.COL2 OR T1.COL2 IS NULL OR T2.COL2 IS NULL) AND
(T1.COL3 = T2.COL3 OR T1.COL3 IS NULL OR T2.COL3 IS NULL) AND
(T1.COL4 = T2.COL4 OR T1.COL4 IS NULL OR T2.COL4 IS NULL))TAB3;
SQL Fiddle1
SQL Fiddle2