The shop stores prices in kopecks in the price_cents column and the tax rate in tax_rate. The backend assembles the tax-inclusive total — price_cents × (1 + tax_rate) / 100 — and the formula gets copy-pasted across the codebase. Safer to compute it once inside the column itself. Build a products table with id (primary key), price_cents (integer, required), tax_rate (number with two decimals, default 0.20), and an auto-computed column total whose value always equals price_cents × (1 + tax_rate) / 100 and is stored physically. Insert one row with price_cents = 1000 and verify total was computed automatically.