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-- Migration 008: Enhance invoice_lines for tax classification
-- Adds tax_amount, discount_amount, net_total, tax_category
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-- Add tax_amount column (calculated from tax_rate × line_total)
ALTER TABLE invoice_lines
  ADD COLUMN tax_amount DECIMAL(12,3) DEFAULT 0 AFTER tax_rate;

-- Add discount_amount column  
ALTER TABLE invoice_lines
  ADD COLUMN discount_amount DECIMAL(12,3) DEFAULT 0 AFTER tax_amount;

-- Add net_total column (subtotal + tax - discount)
ALTER TABLE invoice_lines
  ADD COLUMN net_total DECIMAL(12,3) DEFAULT 0 AFTER discount_amount;

-- Add tax_category for classification
-- standard = 16%, zero_rated = 0%, exempt = no tax, special = variable rate
ALTER TABLE invoice_lines
  ADD COLUMN tax_category VARCHAR(20) DEFAULT 'standard' AFTER net_total;

-- Backfill existing data: calculate tax_amount from line_total * tax_rate
UPDATE invoice_lines
SET tax_amount = ROUND(line_total * tax_rate, 3)
WHERE tax_amount = 0 AND line_total > 0;

-- Backfill: net_total = line_total + tax_amount - discount
UPDATE invoice_lines
SET net_total = ROUND(line_total + tax_amount - discount_amount, 3)
WHERE net_total = 0 AND line_total > 0;

-- Classify zero-rated items
UPDATE invoice_lines
SET tax_category = 'zero_rated'
WHERE tax_rate = 0 OR tax_rate IS NULL;

-- Classify standard rate items
UPDATE invoice_lines
SET tax_category = 'standard'
WHERE tax_rate = 0.16;