我有一个有两列的表格:
+---------+--------+
| keyword | color |
+---------+--------+
| foo | red |
| bar | yellow |
| fobar | red |
| baz | blue |
| bazbaz | green |
+---------+--------+
我需要在 PostgreSQL 中进行某种 one-hot 编码和转换表:
+---------+-----+--------+-------+------+
| 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 |
+---------+-----+--------+-------+------+
如何仅使用 SQL 进行此转换?
如果我理解正确的话,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
tablefunc
扩展和 COALESCE()
来填充所有 NULL 字段:
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=#
如果您只是为了获得
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=#
要在具有大量列的表上使用此代码,请使用 Python 生成查询:
1) 创建一个包含您想要作为列名称的唯一变量的列表,并将其导入到 Python,如下所示:
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
您可以运行相同的操作,但以编程方式创建案例。 在您的示例中,您有一个简短的颜色列表,但如果您有许多不同的类别,您可以运行类似的命令:
SET SESSION group_concat_max_len = 10000;
SET @cases = NULL;
SELECT
GROUP_CONCAT(
DISTINCT
CONCAT(
0xd0a,
"COUNT(CASE WHEN color = '",
color,
"' THEN 1 END) AS ",
color
)
SEPARATOR ','
) INTO @cases
FROM table_name t;
SET @sql = CONCAT(
"SELECT
t.keyword,",
@cases,
"
FROM table_name t
GROUP BY keyword
"
);
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;