How can I select a different column from each row? - sql

Question

I have a table which has three columns with three records. How can I select first value of the first column, second value of the second column, third value of the third column?
table
============
test_tab
id1 id2 id2
=== === ====
100 400 700
200 500 800
300 600 900
Required output like :
100 500 900
How can I achieve this by using Oracle SQL or PL/SQL?

Answer 1

First of all, How you would identify which row is the first and which one is second?
Oracle does not guarantee the order of the records, it must be ordered using order by clause explicitly else it will be, we can say random order (The default query output)
With your test data and result, You can use the following query:
Note: I am considering the third column as ID3 and ordering the rows using ID1
SELECT
MAX(CASE RN
WHEN 1 THEN ID1
END) AS ID1,
MAX(CASE RN
WHEN 2 THEN ID2
END) AS ID2,
MAX(CASE RN
WHEN 3 THEN ID3
END) AS ID3
FROM
(
SELECT
ID1,
ID2,
ID3,
ROW_NUMBER() OVER(
ORDER BY
ID1
) RN
FROM
TEST_TAB
);
Cheers!!

Answer 2

You will need a means of establishing what "first" means.
In Oracle, here is one way to address the example from your question:
create table test_tab(
id1 integer,
id2 integer,
id3 integer
);
insert into test_tab values(100,400,700);
insert into test_tab values(200,500,800);
insert into test_tab values(300,600,900);
commit;
select sum(decode(pos, 1, id1, null)) id1,
sum(decode(pos, 2, id2, null)) id2,
sum(decode(pos, 3, id3, null)) id3
from(
-- this subquery produces a rank column for the whole dataset
-- with an explicit order
select id1, id2, id3, rank() over (order by id1, id2, id3) pos from TEST_TAB
);
In this implementation, the subquery is used to establish an ordering of the rows, adding a new pos column based on the rank() function.
The sum(decode(pos, 3, id3, null)) construct is an Oracle idiom for picking one specific row (row 3 in this case) while ignoring the others.
Basically, for your three rows, the decode will result in NULL for any row but the one with the specified number, so the expression for id3 will only have a non-null value for the third row, hence the sum over the group will be equal to id3 in row 3.
There are many ways to do it, this is just one, and you will likely need to make some adjustments to this implementation for it to work properly in your real code.