CREATE TABLE table_name_81 (ends INT, nat VARCHAR, transfer_window VARCHAR)

What is the average end year of the player from swe and a summer transfer window?
SELECT AVG(ends) FROM table_name_81 WHERE nat = "swe" AND transfer_window = "summer"
SELECT AVG("ends") FROM "table_name_81" WHERE "nat" = "swe" AND "summer" = "transfer_window"