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_15 (nat VARCHAR, name VARCHAR, transfer_window VARCHAR, type VARCHAR)

### Question ###
Marcell Jansen with a transfer window of summer, and of transfer as type is from what nation?

### Accurate SQL ###

SELECT nat FROM table_name_15 WHERE transfer_window = "summer" AND type = "transfer" AND name = "marcell jansen"