Here’s how to get data from specific table within past 20 months of current data.

The problem is that you need to get exactly the start of first month in the same period in the criteria you specified. in below MySQL query, you can achieve that easily.

SELECT * FROM `table`
WHERE `date` BETWEEN DATE_SUB(DATE_SUB(CURDATE(), INTERVAL 20 MONTH), INTERVAL DAY(DATE_SUB(CURDATE(), INTERVAL 20 MONTH)) -1 DAY) AND CURDATE()