Transform table to one-hot-encoding of single column value
If I correctly understand, you need conditional aggregation:
select keyword,
count(case when color = 'red' then 1 end) as red,
count(case when color = 'yellow' then 1 end) as yellow
-- another colors here
from t
group by keyword
Another way to achieve the goal in your test case using tablefunc
extension and COALESCE()
to fill all NULL fields:
postgres=# create table t(keyword varchar,color varchar);
CREATE TABLE
postgres=# insert into t values ('foo','red'),('bar','yellow'),('fobar','red'),('baz','blue'),('bazbaz','green');
INSERT 0 5
postgres=# SELECT keyword, COALESCE(red,0) red,
COALESCE(blue,0) blue, COALESCE(green,0) green,
COALESCE(yellow,0) yellow
FROM crosstab(
$$select keyword, color, COALESCE('1',0) as onehot from test01
group by 1, 2 order by 1, 2$$,
$$select distinct color from test01 order by 1$$)
AS result(keyword varchar, blue int, green int, red int, yellow int);
keyword | red | blue | green | yellow
---------+-----+------+-------+--------
bar | 0 | 0 | 0 | 1
baz | 0 | 1 | 0 | 0
bazbaz | 0 | 0 | 1 | 0
fobar | 1 | 0 | 0 | 0
foo | 1 | 0 | 0 | 0
(5 rows)
postgres=#
And if you just to obtain the result under psql
:
postgres=# select keyword, color, COALESCE('1',0) as onehot from t
--group by 1, 2 order by 1, 2
\crosstabview keyword color
keyword | red | yellow | blue | green
---------+-----+--------+------+-------
foo | 1 | | |
bar | | 1 | |
fobar | 1 | | |
baz | | | 1 |
bazbaz | | | | 1
(5 rows)
postgres=#
To use this code on a table with a high number of columns, use Python to generate your queries:
1) Create a list with unique variables that you want to have as your column names and import this to Python, say as: list
.
for item in list:
print('count(case when item=' +str(item)+ 'then 1 end) as is_'+str(item)+',')
2) Copy the output (minus the last comma on the last row)
3) Then:
select keyword,
OUTPUT FROM PYTHON
from t
group by keyword