Transform table to one-hot encoding for many rows

I have a SQL table of the following format:

ID  Cat
1   A   
1   B
1   D
1   F
2   B
2   C
2   D
3   A
3   F

Now, I want to create a table with one ID per row, and multiple Cat’s in a row. My desired output looks as follows:

ID  A  B  C  D  E  F
1   1  1  0  1  0  1
2   0  1  1  1  0  0
3   1  0  0  0  0  1

I have found:

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

However, I have more than 1000 Cat’s, so I am looking for code to write this automatically, rather than manually. Who can help me with this?

Answer

First let me transform the data you pasted into an actual table:

WITH data AS (
  SELECT REGEXP_EXTRACT(data2, '[0-9]') id, REGEXP_EXTRACT(data2, '[A-Z]') cat
  FROM (
    SELECT SPLIT("""1   A   
    1   B
    1   D
    1   F
    2   B
    2   C
    2   D
    3   A
    3   F""", 'n') AS data1
  ), UNNEST(data1) data2
)

SELECT * FROM data

(try sharing a table next time)

Now we can do some manual 1-hot encoding:

SELECT id 
 , MAX(IF(cat='A',1,0)) cat_A
 , MAX(IF(cat='B',1,0)) cat_B
 , MAX(IF(cat='C',1,0)) cat_C
FROM data
GROUP BY id

Now we want to write a script that will automatically create the columns we want:

SELECT STRING_AGG(FORMAT("MAX(IF(cat='%s',1,0))cat_%s", cat, cat), ', ') 
FROM (
  SELECT DISTINCT cat
  FROM data
  ORDER BY 1
)

That generates a string that you can copy paste into a query, that 1-hot encodes your arrays/rows:

SELECT id
,
MAX(IF(cat='A',1,0))cat_A, MAX(IF(cat='B',1,0))cat_B, MAX(IF(cat='C',1,0))cat_C, MAX(IF(cat='D',1,0))cat_D, MAX(IF(cat='F',1,0))cat_F
FROM data
GROUP BY id

And that’s exactly what the question was asking for. You can generate SQL with SQL, but you’ll need to write a new query using that result.