MySQL Query to calculate the Previous Month

Here you go, use this to get the date between the 1st of last month and the last of last month in MySQL:

... order_placed_date BETWEEN DATE_FORMAT(NOW() - INTERVAL 1 MONTH, '%Y-%m-01 00:00:00')
AND DATE_FORMAT(LAST_DAY(NOW() - INTERVAL 1 MONTH), '%Y-%m-%d 23:59:59')