Need to clean out a product file? Old crusty sale prices sitting around from 6 years ago? Run these mysql commands to make the nothing but a faint memory:
DELETE FROM wp_postmeta WHERE meta_key = '_sale_price';
DELETE FROM wp_postmeta WHERE meta_key = '_sale_price_dates_to';
DELETE FROM wp_postmeta WHERE meta_key = '_sale_price_dates_from';
As with all database manipulation. Make sure you back it up before you potentially break it. You’ll thank me later.
Source for this handy information: https://www.webdesign101.net/remove-woocommerce-sale-prices/