Preserve backfill emails

backend/src/main/resources/db/migration/V9__backfill_user_accounts.sql

@@ -2,7 +2,15 @@ INSERT INTO users (username, password, email, fullName, phone, role, active, tok
 SELECT
     CONCAT('customer_', c.customerId) AS username,
     '$2a$10$mE0D/HrnCuqFeEqMy0NJwuy2jkoRYjQ7GrKcc/7QQ0r2AqnZTvyGq' AS password,
-    CONCAT('customer_', c.customerId, '@petshop.local') AS email,
+    CASE
+        WHEN c.email IS NOT NULL
+         AND c.email <> ''
+         AND (SELECT COUNT(*) FROM customer c2 WHERE c2.email = c.email) = 1
+         AND NOT EXISTS (SELECT 1 FROM employee e2 WHERE e2.email = c.email)
+         AND NOT EXISTS (SELECT 1 FROM users u WHERE u.email = c.email)
+        THEN c.email
+        ELSE CONCAT('customer_', c.customerId, '@petshop.local')
+    END AS email,
     CONCAT(c.firstName, ' ', c.lastName) AS fullName,
     CONCAT('200-000-', LPAD(c.customerId, 4, '0')) AS phone,
     'CUSTOMER' AS role,
@@ -20,7 +28,15 @@ INSERT INTO users (username, password, email, fullName, phone, role, active, tok
 SELECT
     CONCAT('employee_', e.employeeId) AS username,
     '$2a$10$mE0D/HrnCuqFeEqMy0NJwuy2jkoRYjQ7GrKcc/7QQ0r2AqnZTvyGq' AS password,
-    CONCAT('employee_', e.employeeId, '@petshop.local') AS email,
+    CASE
+        WHEN e.email IS NOT NULL
+         AND e.email <> ''
+         AND (SELECT COUNT(*) FROM employee e2 WHERE e2.email = e.email) = 1
+         AND NOT EXISTS (SELECT 1 FROM customer c2 WHERE c2.email = e.email)
+         AND NOT EXISTS (SELECT 1 FROM users u WHERE u.email = e.email)
+        THEN e.email
+        ELSE CONCAT('employee_', e.employeeId, '@petshop.local')
+    END AS email,
     CONCAT(e.firstName, ' ', e.lastName) AS fullName,
     CONCAT('300-000-', LPAD(e.employeeId, 4, '0')) AS phone,
     CASE