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 stadium (country VARCHAR, opening_year INTEGER)

### Question ###
Which countries do not have a stadium that was opened after 2006?

### Accurate SQL ###

SELECT country FROM stadium EXCEPT SELECT country FROM stadium WHERE opening_year > 2006