Thank you for this document: http://blog.sina.com.cn/s/blog_499740cb0100ugs7.html
After mysql opens the transaction, it will lock the row. For update, the row lock will be performed. At this time, if a query select is performed, a copy will be saved. If another transaction submits the data, the current transaction will not be able to query the new data. , but to query the previous copy. If you want to query new data, the query will not be executed when the transaction is started, and the latest data will be obtained.