How to transpose row to column in Postgres 9.5? - sql

Question

I have this table in input, it contains always only three rows.
| data |
--------
| X |
| Y |
| Z |
And I want this output:
| data1| data2 | data3 |
-------+-------+-------+
| X | Y | Z |
I have tried to use the crosstab function, but as far as I understand it need more information, like a category column and a row_name column. I don't have them.
Is possible to transpose this table?

Answer 1

You don't need a crosstab function to do this, use just a simple PIVOT query:
SELECT max( case rn when 1 then data end ) as data1,
max( case rn when 2 then data end ) as data2,
max( case rn when 3 then data end ) as data3
FROM (
SELECT *,
row_number() over ( ORDER BY data ) rn
FROM table1
) x
Demo: http://sqlfiddle.com/#!15/bead8/4
There is one pitfall in this query you need to think about.
The database table is by definition an unordered set ot tuples, and hardly any database guarantees an ordering of the rows unless an ORDER BY clause is specified in the SELECT statement that queries the table.
Because of this the query uses ORDER BY data clause to order rows in such a way, that X will be put to data1 column, Y to data2 and Z to data3, in this order (becauce X < Y < Z).
You need to change this clause If you need to use some other order (or maybe some other column of this table to determine this order).

Answer 2

For fixed columns/rows count:
select
data[1] as data1,
data[2] as data2,
data[3] as data3
from
(select array_agg(data) as data from t) as t;
For variable columns/rows count (only one from many possibilities):
create function prepare_statement(in p_name text, in p_body text) returns void as $$
declare
s text;
begin
s := 'prepare ' || p_name || ' as ' || p_body;
execute s;
return;
end; $$ language plpgsql;
and then:
select prepare_statement('foo', (
select
'select ' ||
string_agg('data['||i||'] as data'||i, ', ') ||
' from (select array_agg(data) as data from t) as t'
from generate_series(1, (select count(*) from t)) n(i))
);
execute foo;
-- deallocate foo; -- to deallocate previously prepared statement
Read more about
arrays
array_agg function
prepare/execute/deallocate statements