关于knock的语法knock in和 knock at有什么区别

knock at敲打某东西,He knocked at the melon to judge if it was fully ripe.
knock in to把东西敲（撞）进.,He knocked me in to the water.
后面有door时敲门用knock at,knock on都可以
后面没有door时用knock
He came in without knocking.他没敲门就进来了
I knocked on (at)the door,but there was no answer.我敲了门,但没有回应