once a dog chased me which frightened me a litte.請問這句中which前是否漏了逗號?因為只有在限定關代中才能用which代表前面一整件事.是嗎?謝謝!?

with the rel pron as the subject of the clause:-,which frightened me.
in non-defining clause that refer to a clause="once a dog chased me"not to a special dog, because only one dog.The meaning is still complete without it.Hence,without two commas=non-defining, although only one dog=tends to become a defining.
However, a defining adjective clause is a necessary in the sentence because the meaning  will be unclear without it.(if there are two or more dogs)

Nope
Do not need