ALTER TABLE rewards
  ADD COLUMN description VARCHAR(500) NOT NULL DEFAULT '' AFTER stock;

UPDATE rewards
SET stock = CASE id
  WHEN 'reward_001' THEN 30
  WHEN 'reward_002' THEN 50
  WHEN 'reward_003' THEN 20
  WHEN 'reward_004' THEN 80
  ELSE stock
END,
description = CASE id
  WHEN 'reward_001' THEN '适用于校区公开试听课，审核通过后生成课程券码，30 天内到店核销。'
  WHEN 'reward_002' THEN '由课程顾问提供一次学习规划沟通，适合新生测评后使用。'
  WHEN 'reward_003' THEN '报名正价课程时抵扣 100 元，不可叠加现金活动使用。'
  WHEN 'reward_004' THEN '兑换后安排一次阶段测评报告解读，帮助家长了解学习状态。'
  ELSE description
END;