SQL writing problem to calculate the ring ratio of the current month and the previous month

Excuse me , I want to compare the sales volume of each month with that of last month , How to write this if it comes out of the loop sql Sentence? ？

It's only two months now , But there will be 7 Of the month ,8 Of the month , I want to cycle through something like 8 Monthly ratio 7 How much more month on month ,

For example, there should be more ring comparison columns in the back , Do you want to do it in a loop , Ask for advice ~~