Merge data from multiple columns into one - sql

Question

I have three columns X, Y & Z. I want to do a select statement that returns only one column.
For example for the following rows:
X Y Z
1 0 0
0 1 0
0 0 1
I want to return one column A:
A
X
Y
Z
So, wherever there is one, the column should return a string corresponding to the column name where it is one..
I don't have rights to create a new column in the database and then update it using where condition. So I was wondering if it could be done inside a SELECT statement

Answer 1

Without dealing with multiple 1 values in one row:
select case
when x = 1 then 'X'
when y = 1 then 'Y'
when z = 1 then 'Z'
end as A
from the_table;
If you are using Postgres and have a primary key column on that table, you can use JSON functions to make this dynamic and not hardcode the column names in the query.
Test data setup:
create table the_table (id integer, x int, y int, z int);
insert into the_table
(id,x,y,z)
values
(1, 1, 0, 0),
(2, 0, 1, 0),
(3, 0, 0, 1),
(4, 0, 1, 1),
(5, 0, 0, 0);
Then using this query:
select t.id, string_agg(k.col,'' order by k.col) as non_zero_columns
from the_table t,
jsonb_each(to_jsonb(t) - 'id') as k (col, val)
where k.val = '1'
group by id
order by id;
Will return this result:
id | non_zero_columns
---+-----------------
1 | x
2 | y
3 | z
4 | yz
Note that the row with ID=5 is not returned because all columns are zero.

Answer 2

If you have multiple columns with 1s in one row:
select ((case when x = 1 then 'X' else '' end) ||
(case when y = 1 then 'Y' else '' end) ||
(case when z = 1 then 'Z' else '' end)
) as A
from the_table;
Note that || is the ANSI standard operator for string concatenation. Some databases use other methods for concatenating strings.

Answer 3

if you are using ms sq-server, you can use UNPIVOT
DECLARE #MyTable TABLE (X INT, Y INT, Z INT)
INSERT INTO #MyTable VALUES
(1 ,0, 0 ),
(0 ,1, 0 ),
(0 ,0, 1 )
SELECT DISTINCT A FROM
(select * from #MyTable UNPIVOT( V FOR A IN ([X], [Y], [Z])) UNPVT) UP
WHERE V = 1
Result:
A
---------
X
Y
Z

Answer 4

You can use the INFORMATION_SCHEMA with find_in_set(str, strlist) function (MySql):
-- the schema
create table `docs` (
`X` int not null,
`Y` int not null,
`Z` int not null
);
insert into `docs` (`X`, `Y`, `Z`) values
(1, 0, 0),
(0, 1, 0),
(0, 0, 1);
-- the query
select column_name as A, concat(x, y, z)
from docs
join INFORMATION_SCHEMA.COLUMNS
on (ordinal_position = find_in_set('1', concat(x, ',', y, ',', z)))
where table_name like 'docs';
Here, the SQL Fiddle