CREATE TABLE table_name_50 (year NUMBER, event VARCHAR2, opponent VARCHAR2)

what's the earliest year that the wimbledon opponent was zheng jie?
SELECT MIN(year) FROM table_name_50 WHERE event = "wimbledon" AND opponent = "zheng jie"
SELECT MIN("year") FROM "table_name_50" WHERE "event" = "wimbledon" AND "opponent" = "zheng jie"