diff --git a/prisma/migrations/19_add_user_id_mapping/migration.sql b/prisma/migrations/19_add_user_id_mapping/migration.sql
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+-- Migration: Add User ID Mapping
+-- Description: Add user_id column to session table for unified user identification
+-- Date: 2025-11-05
+
+-- Add user_id column to session table
+ALTER TABLE session ADD COLUMN IF NOT EXISTS user_id VARCHAR(36);
+
+-- Add index for user_id lookups
+CREATE INDEX IF NOT EXISTS idx_session_user_id ON session(user_id);
+
+-- Add composite index for visitor_id and user_id
+CREATE INDEX IF NOT EXISTS idx_session_visitor_user ON session(visitor_id, user_id);
+
+-- Create user_mapping table for tracking visitor_id to user_id relationships
+CREATE TABLE IF NOT EXISTS user_mapping (
+    id SERIAL PRIMARY KEY,
+    visitor_id VARCHAR(36) NOT NULL,
+    user_id VARCHAR(36) NOT NULL,
+    first_seen_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+    last_seen_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+    session_count INTEGER DEFAULT 1,
+    UNIQUE(visitor_id, user_id)
+);
+
+-- Add indexes for user_mapping
+CREATE INDEX IF NOT EXISTS idx_user_mapping_visitor ON user_mapping(visitor_id);
+CREATE INDEX IF NOT EXISTS idx_user_mapping_user ON user_mapping(user_id);
+CREATE INDEX IF NOT EXISTS idx_user_mapping_last_seen ON user_mapping(last_seen_at);
+
+-- Add comments
+COMMENT ON COLUMN session.user_id IS 'WordPress user ID for logged-in users';
+COMMENT ON TABLE user_mapping IS 'Maps visitor IDs to user IDs for unified identification';
+COMMENT ON COLUMN user_mapping.visitor_id IS 'Umami visitor ID (anonymous)';
+COMMENT ON COLUMN user_mapping.user_id IS 'WordPress user ID (logged-in)';
+COMMENT ON COLUMN user_mapping.session_count IS 'Number of sessions for this visitor-user pair';
+