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 table_name_88 ( nation VARCHAR, world_rank VARCHAR, birth_date VARCHAR )

### Question ###
what is the nation when the world rank is 6 and the birth date is 1971-07-31?

### Accurate SQL ###

SELECT nation FROM table_name_88 WHERE world_rank = "6" AND birth_date = "1971-07-31"