[Plugin Development] How to add more conditions to kong.db.<tbl>:select()

I am new to develop kong plugins, suppose I have a database table created and I can already use kong.db.<tbl_name>:select() to select the record by ID. However, I’d like to add more customized WHERE clause like “deleted_at=null” or any other conditions to filter out my records, how should I do that? Thank you.