问题描述
我正在尝试使用 GROUP ... WITH ROLLUP 来获取多个分组值的一系列总计，但没有小计。因此，在我人为的示例中，我有一个表 fruits，如下所示：
SELECT * FROM fruits ORDER BY fruit;
+----+----------+------+-------+
| id | fruit | size | price |
+----+----------+------+-------+
| 1 | apple | 3 | 4.99 |
| 2 | apple | 4 | 5.99 |
| 3 | apple | 6 | 2.99 |
| 8 | apple | 3 | 3.00 |
| 9 | apple | 4 | 6.50 |
| 4 | banana | 2 | 3.50 |
| 5 | banana | 3 | 2.10 |
| 6 | banana | 1 | 8.99 |
| 7 | banana | 7 | 6.99 |
| 10 | banana | 3 | 3.50 |
| 14 | banana | 7 | 8.00 |
| 11 | cucumber | 1 | 1.50 |
| 12 | cucumber | 1 | 2.30 |
| 13 | cucumber | 2 | 3.30 |
+----+----------+------+-------+
假设我想按 fruit 和 size 对水果表进行分组，并在底部显示平均价格。我可以使用 GROUP WITH ROLLUP 来实现这一点：
SELECT fruit,size,AVG(price) FROM fruits GROUP BY fruit,size WITH ROLLUP;
+----------+------+------------+
| fruit | size | AVG(price) |
+----------+------+------------+
| apple | 3 | 3.995000 |
| apple | 4 | 6.245000 |
| apple | 6 | 2.990000 |
| apple | NULL | 4.694000 |
| banana | 1 | 8.990000 |
| banana | 2 | 3.500000 |
| banana | 3 | 2.800000 |
| banana | 7 | 7.495000 |
| banana | NULL | 5.513333 |
| cucumber | 1 | 1.900000 |
| cucumber | 2 | 3.300000 |
| cucumber | NULL | 2.366667 |
| NULL | NULL | 4.546429 |
+----------+------+------------+
但我不想要所有这些小计行...我只想要最后一行，其中 fruit 和 size 都显示为 NULL。换句话说，这是：
+----------+------+------------+
| fruit | size | AVG(price) |
+----------+------+------------+
| apple | 3 | 3.995000 |
| apple | 4 | 6.245000 |
| apple | 6 | 2.990000 |
| banana | 1 | 8.990000 |
| banana | 2 | 3.500000 |
| banana | 3 | 2.800000 |
| banana | 7 | 7.495000 |
| cucumber | 1 | 1.900000 |
| cucumber | 2 | 3.300000 |
| NULL | NULL | 4.546429 |
+----------+------+------------+
我无法访问 MysqL 8.0，因此我无法使用 GROUPING() 函数 described in the docs 来确定行中的值是表示小计还是总计。所以我想我必须过滤掉 NULL 值。
我试着像这样适应this answer to a similar question：
SELECT fruit,size WITH ROLLUP HAVING size IS NOT NULL or fruit IS NULL;
+----------+------+------------+
| fruit | size | AVG(price) |
+----------+------+------------+
| apple | 3 | 3.995000 |
| apple | 4 | 6.245000 |
| apple | 6 | 2.990000 |
| banana | 1 | 8.990000 |
| banana | 2 | 3.500000 |
| banana | 3 | 2.800000 |
| banana | 7 | 7.495000 |
| cucumber | 1 | 1.900000 |
| cucumber | 2 | 3.300000 |
+----------+------+------------+
不过，正如您所看到的，这不会产生我想要的结果 - 它只是完全砍掉了总计。我知道我只想获取大小为非空且水果为空或不为空的行，但是我尝试测试 NULL 的各种方法没有按预期工作：
SELECT fruit,AVG(price),fruit IS NULL,fruit IS NOT NULL,ISNULL(fruit),fruit = "" FROM fruits GROUP BY fruit,size WITH ROLLUP;
+----------+------+------------+---------------+-------------------+---------------+------------+
| fruit | size | AVG(price) | fruit IS NULL | fruit IS NOT NULL | ISNULL(fruit) | fruit = "" |
+----------+------+------------+---------------+-------------------+---------------+------------+
| apple | 3 | 3.995000 | 0 | 1 | 0 | 0 |
| apple | 4 | 6.245000 | 0 | 1 | 0 | 0 |
| apple | 6 | 2.990000 | 0 | 1 | 0 | 0 |
| apple | NULL | 4.694000 | 0 | 1 | 0 | 0 |
| banana | 1 | 8.990000 | 0 | 1 | 0 | 0 |
| banana | 2 | 3.500000 | 0 | 1 | 0 | 0 |
| banana | 3 | 2.800000 | 0 | 1 | 0 | 0 |
| banana | 7 | 7.495000 | 0 | 1 | 0 | 0 |
| banana | NULL | 5.513333 | 0 | 1 | 0 | 0 |
| cucumber | 1 | 1.900000 | 0 | 1 | 0 | 0 |
| cucumber | 2 | 3.300000 | 0 | 1 | 0 | 0 |
| cucumber | NULL | 2.366667 | 0 | 1 | 0 | 0 |
| NULL | NULL | 4.546429 | 0 | 0 | 0 | NULL |
+----------+------+------------+---------------+-------------------+---------------+------------+
这些结果都没有任何意义。我会假设 fruit is NULL 或 ISNULL(fruit) 会在水果的值实际上为 NULL 时返回 1。但他们没有。所以我想知道这是否与空字符串和 null 之间的区别有关……但我不明白为什么，当我测试 fruit = "" 时，当水果为 NULL 时它返回 NULL。>
我应该使用什么条件来测试 NULL，以便我可以从我的表中排除我想要的结果？
解决方法
将您的聚合查询用作子查询和过滤器：
SELECT *
FROM (
SELECT fruit,size,AVG(price) avg_price
FROM fruits
GROUP BY fruit,size
WITH ROLLUP
) t
WHERE (fruit IS NOT NULL AND size IS NOT NULL)
OR (fruit IS NULL AND size IS NULL)
你也可以不用子查询，使用 HAVING 子句：
SELECT fruit,AVG(price) avg_price
FROM fruits
GROUP BY fruit,size
WITH ROLLUP
HAVING (fruit IS NOT NULL AND size IS NOT NULL)
OR (fruit IS NULL AND size IS NULL)
参见demo。
,不需要子查询。您可以使用 having 子句：
select fruit,avg(price)
from fruits
group by fruit,size with rollup
having fruit is not null and size is not null or
fruit is null and size is null;
还有其他表达having的方式，例如：
having (fruit is not null) = (size is not null)
Here 是一个 dbfiddle。