Hi @wu cong? , thank you for reaching out.
I believe this is in reference to the below community question. Please access the below link for a complete solution.
https://community.trifacta.com/s/question/0D53j00007zypsKCAQ/i-have-one-table-which-has-field-acfield-a-field-b-will-be-an-unique-key-and-the-data-will-be-sorted-by-field-a-and-fieldbnow-i-want-to-get-the-min-field-c-by-each-record-before-the-current-record
Do let me know if you have any additional queries.
Best,
Apeksha