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You will probably need to go in reverse order to prevent moving a lower priced item into a medium price and then upping it when increasing the medium.
In other words, start with the higher and work down to the <=10GBP. Doing a 9 to start would move it to 18 which would again be changed when you did the 11-50.
Try using this (NOT TESTED)Next, you'd go to the 11-50 prices and lastly, the 0-10 items.UPDATE products
SET products_price = products_price * 1.35
WHERE products_price => "51" AND products_price <= "100"
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