Hi @Kai_Zhu welcome to the Coda community! 
Upsert is an amalgam of Insert and Update. Its best used when you are uncertain if the row exists.
If you try to Update a row that doesn’t exist, the zap will fail. If you try to Insert a row that already exists, the zap will fail.
Upsert will always work … but sometimes you do want it to fail. It gives you more flexibility depending on the use case.
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Alright, so this is just a non-English speaker’s misunderstanding.
Thanks!
