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 ( city VARCHAR, opening_year INTEGER )

### Question ###
How many cities have a stadium that was opened before the year of 2006?

### Accurate SQL ###

SELECT COUNT(DISTINCT city) FROM stadium WHERE opening_year < 2006