将 table 转换为单列值的单热编码

Question

我有一个包含两列的 table：

+---------+--------+
| keyword | color  |
+---------+--------+
| foo     | red    |
| bar     | yellow |
| fobar   | red    |
| baz     | blue   |
| bazbaz  | green  |
+---------+--------+

我需要进行某种一次性编码并将 PostgreSQL 中的 table 转换为：

+---------+-----+--------+-------+------+
| keyword | red | yellow | green | blue |
+---------+-----+--------+-------+------+
| foo     |   1 |      0 |     0 |    0 |
| bar     |   0 |      1 |     0 |    0 |
| fobar   |   1 |      0 |     0 |    0 |
| baz     |   0 |      0 |     0 |    1 |
| bazbaz  |   0 |      0 |     1 |    0 |
+---------+-----+--------+-------+------+

是否可以只用SQL？关于如何开始的任何提示？

Answer 1

如果我没理解错的话，你需要条件聚合：

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

Answer 2

要在具有大量列的 table 上使用此代码，请使用 Python 生成您的查询：

1) 创建一个包含您希望作为列名的唯一变量的列表，并将其导入 Python，如：list.

for item in list:
 print('count(case when item=' +str(item)+ 'then 1 end) as is_'+str(item)+',')

2) 复制输出（减去最后一行的最后一个逗号）

3) 然后：

select keyword,

OUTPUT FROM PYTHON

from t
group by keyword

Answer 3

使用 tablefunc extension and COALESCE() to fill all NULL fields 实现测试用例目标的另一种方法：

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=#

而如果你只是为了获得psql下的结果：

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=#

将 table 转换为单列值的单热编码

Transform table to one-hot-encoding of single column value

sql

postgresql

crosstab