Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE mill (id VARCHAR, architect_id VARCHAR, built_year INTEGER)
TABLE: CREATE TABLE architect (id VARCHAR, architect_id VARCHAR, built_year INTEGER)

### Question ###
How many architects haven't built a mill before year 1850?

### Accurate SQL ###

SELECT COUNT(*) FROM architect WHERE NOT id IN (SELECT architect_id FROM mill WHERE built_year < 1850)