关于索引的最左前缀法则问题

[goodoh]

佬们 在学习索引时实操发现似乎没有遵循最左前缀法则也使用了索引，为啥

CREATE INDEX idx_department_did_dname_location ON department(did,dname,location);

EXPLAIN SELECT dname FROM department WHERE  location = '图书馆';