How to create dummy variable columns for thousands of categories in Google BigQuery?

Question

I have a simple table with 2 columns: UserID and Category, and each UserID can repeat with a few categories, like so:

UserID   Category
------   --------
1           


        

Answer 1

You can use below "technic"

First run query #1. It produces the query (query #2) that you need to run to get result you need. Please, still consider Mosha's comments before going "wild" with thousands categories :o)

Query #1:

SELECT 'select UserID, ' + 
   GROUP_CONCAT_UNQUOTED(
    'sum(if(category = "' + STRING(category) + '", 1, 0)) as ' + STRING(category)
   ) 
   + ' from YourTable group by UserID'
FROM (
  SELECT category 
  FROM YourTable  
  GROUP BY category
)

Resulted will be like below - Query #2

SELECT
  UserID,
  SUM(IF(category = "A", 1, 0)) AS A,
  SUM(IF(category = "B", 1, 0)) AS B,
  SUM(IF(category = "C", 1, 0)) AS C
FROM
  YourTable
GROUP BY
  UserID

of course for three categories - you could do it manually, but for thousands it will definitelly will make day for you!!

Result of query #2 will looks as you expect:

UserID  A   B   C    
1       1   1   0    
2       0   0   1    
3       1   1   1