how to – mySQL – Date Difference and Flags

I am very new to mySQL and currently working on a table with three columns: trx_id, user_id, last_activity. (Churn Analysis)

tbl_activity:

The table capture activity of users. I am finding it difficulty in performing two tasks.

1) I would like to see two new columns through SQL query

• date difference between subsequent transactions.
• flag based on condition > 30 days.

desired table:

2) One of the objectives of this study is to identify when (date) a customer churned. Ideally in my case it would be the 31st day since last activity. Any way to arrive at this date?

I am new to SQL learning and finding it difficult to address SQL queries for the above tasks. Thank you in advance.

Source: Stackoverflow