和( col + col 2)如果col 1包含空

Question

Input: 
Boxes table:
+--------+----------+-------------+--------------+
| box_id | chest_id | apple_count | orange_count |
+--------+----------+-------------+--------------+
| 2      | null     | 6           | 15           |
| 18     | 14       | 4           | 15           |
| 19     | 3        | 8           | 4            |
| 12     | 2        | 19          | 20           |
| 20     | 6        | 12          | 9            |
| 8      | 6        | 9           | 9            |
| 3      | 14       | 16          | 7            |
+--------+----------+-------------+--------------+
Chests table:
+----------+-------------+--------------+
| chest_id | apple_count | orange_count |
+----------+-------------+--------------+
| 6        | 5           | 6            |
| 14       | 20          | 10           |
| 2        | 8           | 8            |
| 3        | 19          | 4            |
| 16       | 19          | 19           |
+----------+-------------+--------------+
Output: 
+-------------+--------------+
| apple_count | orange_count |
+-------------+--------------+
| 151         | 123          |
+-------------+--------------+
Explanation: 
box 2 has 6 apples and 15 oranges.
box 18 has 4 + 20 (from the chest) = 24 apples and 15 + 10 (from the chest) = 25 oranges.
box 19 has 8 + 19 (from the chest) = 27 apples and 4 + 4 (from the chest) = 8 oranges.
box 12 has 19 + 8 (from the chest) = 27 apples and 20 + 8 (from the chest) = 28 oranges.
box 20 has 12 + 5 (from the chest) = 17 apples and 9 + 6 (from the chest) = 15 oranges.
box 8 has 9 + 5 (from the chest) = 14 apples and 9 + 6 (from the chest) = 15 oranges.
box 3 has 16 + 20 (from the chest) = 36 apples and 7 + 10 (from the chest) = 17 oranges.
Total number of apples = 6 + 24 + 27 + 27 + 17 + 14 + 36 = 151
Total number of oranges = 15 + 25 + 8 + 28 + 15 + 15 + 17 = 123

My answer:
SELECT 
SUM(b.apple_count +c.apple_count,0) AS apple_count, # IFNULL
SUM(b.orange_count+c.orange_count,0) AS orange_count
FROM Boxes b
LEFT JOIN Chests c
ON b.chest_id = c.chest_id

The expected answer:
SELECT 
SUM(b.apple_count +COALESCE(c.apple_count,0)) AS apple_count, # IFNULL
SUM(b.orange_count+COALESCE(c.orange_count,0)) AS orange_count
FROM Boxes b
LEFT JOIN Chests c
ON b.chest_id = c.chest_id

我的问题是，为什么我们必须在这个问题上使用COALESCE()或IFNULL()？

当使用SUM()时，0和null之间有什么区别？

我认为SUM()应该忽略MySQL中的null值，将6添加到苹果，15添加到橙色，即使在胸前表中添加了null？

Answer 1

1+0=1

1+ null =空

这就是区别所在。