小店月底盘点：每个分类（category）一共还剩多少件？把同一类的 stock 加起来。结果两列：分类，和这一类的库存总数。
| id | name | category | price | stock |
|---|---|---|---|---|
| 1 | Milk tea | drink | 12 | 30 |
| 2 | Lemon water | drink | 8 | 50 |
| 3 | Cheesecake | cake | 25 | 6 |
| 4 | Cookie | snack | 5 | 80 |
| 5 | Brownie | cake | 18 | 0 |
| 6 | Iced coffee | drink | 15 | 20 |
| 7 | Chips | snack | 7 | 40 |
| 8 | Fruit tart | cake | 28 | 4 |
| id | name | category | price | stock |
|---|---|---|---|---|
| 1 | Milk tea | drink | 12 | 30 |
| 2 | Lemon water | drink | 8 | 50 |
| 3 | Cheesecake | cake | 25 | 6 |
| 4 | Cookie | snack | 5 | 80 |
| 5 | Brownie | cake | 18 | 0 |
| 6 | Iced coffee | drink | 15 | 20 |
| 7 | Chips | snack | 7 | 40 |
| 8 | Fruit tart | cake | 28 | 4 |
-- 在这里写：按 category 分组，用 SUM(stock) 把每组的库存加起来 SELECT category, stock FROM products;
写好了先点「运行」看看结果，没问题再点「提交」。