MySQL Workbench, NULL 的平均值是 returning "0", 我需要它 return NULL

Question

我正在尝试平均我的绘图级别数据。当我在绘图级别执行查询时，它会导致与某些行名称关联的所有行的结果为 "NULL"。当我执行相同的查询尝试对特征进行 AVG 并按行名称分组时，结果显示“0”。我需要它来显示 "NULL" 所以它不会影响我的总平均值。

select 地块级数据的示例代码：

SELECT experiment_name, `year`, location_name, line_name, head_dap, height_cm, yield_bua
FROM data_amba
WHERE experiment_name = 'matspr' AND `year` = 2018 AND location_name = 'Stephen'
GROUP BY line_name, head_dap, height_cm, yield_bua;

结果：

+------------+----------+-----------+-----------+
| line_name  | head_dap | height_cm | yield_bua |
+------------+----------+-----------+-----------+
| ND_GENESIS | NULL     | NULL      | 130.9     |
+------------+----------+-----------+-----------+
| ND_GENESIS | NULL     | NULL      | 131.3     |
+------------+----------+-----------+-----------+
| ND_GENESIS | NULL     | NULL      | 134.3     |
+------------+----------+-----------+-----------+
| PINNACLE   | NULL     | NULL      | 144.9     |
+------------+----------+-----------+-----------+
| PINNACLE   | NULL     | NULL      | 146.6     |
+------------+----------+-----------+-----------+
| PINNACLE   | NULL     | NULL      | 163.9     |
+------------+----------+-----------+-----------+
| S2M177     | NULL     | NULL      | 107.6     |
+------------+----------+-----------+-----------+
| S2M177     | NULL     | NULL      | 142.6     |
+------------+----------+-----------+-----------+
| S2M177     | NULL     | NULL      | 168.6     |
+------------+----------+-----------+-----------+
| S2M178     | NULL     | NULL      | 114.6     |
+------------+----------+-----------+-----------+
| S2M178     | NULL     | NULL      | 132.9     |
+------------+----------+-----------+-----------+
| S2M178     | NULL     | NULL      | 154.6     |
+------------+----------+-----------+-----------+
| S2M179     | NULL     | NULL      | 132.6     |
+------------+----------+-----------+-----------+
| S2M179     | NULL     | NULL      | 137.9     |
+------------+----------+-----------+-----------+
| S2M179     | NULL     | NULL      | 140.6     |
+------------+----------+-----------+-----------+
| S2M180     | NULL     | NULL      | 101.9     |
+------------+----------+-----------+-----------+
| S2M180     | NULL     | NULL      | 147.3     |
+------------+----------+-----------+-----------+
| S2M180     | NULL     | NULL      | 169.6     |
+------------+----------+-----------+-----------+
| S2M181     | NULL     | NULL      | 111.9     |
+------------+----------+-----------+-----------+
| S2M181     | NULL     | NULL      | 125.3     |
+------------+----------+-----------+-----------+
| S2M181     | NULL     | NULL      | 138.9     |
+------------+----------+-----------+-----------+

select 平均值的示例代码：

SELECT experiment_name, `year`, location_name, line_name, AVG(head_dap), AVG(height_cm), AVG(yield_bua)
FROM data_amba
WHERE experiment_name = 'matspr' AND `year` = 2018 AND location_name = 'Stephen'
GROUP BY line_name;

结果：

+------------+---------------+----------------+----------------+
| line_name  | AVG(head_dap) | AVG(height_cm) | AVG(yield_bua) |
+------------+---------------+----------------+----------------+
| ND_GENESIS | 0             | 0              | 132.1667       |
+------------+---------------+----------------+----------------+
| PINNACLE   | 0             | 0              | 151.8          |
+------------+---------------+----------------+----------------+
| S2M177     | 0             | 0              | 139.6          |
+------------+---------------+----------------+----------------+
| S2M178     | 0             | 0              | 134.0333       |
+------------+---------------+----------------+----------------+
| S2M179     | 0             | 0              | 137.0333       |
+------------+---------------+----------------+----------------+
| S2M180     | 0             | 0              | 139.6          |
+------------+---------------+----------------+----------------+
| S2M181     | 0             | 0              | 125.3667       |
+------------+---------------+----------------+----------------+

当对 3 个 "NULL" 值进行平均时，我希望在结果中看到 "NULL" 而不是“0”，因此它不会影响我在下游执行的任何总均值计算。

Answer 1

我能想到的最简单的解决方法是使用 NULLIF 函数 -

SELECT experiment_name
      ,`year`
      ,location_name
      ,line_name
      ,NULLIF(AVG(head_dap), 0)
      ,NULLIF(AVG(height_cm), 0)
      ,NULLIF(AVG(yield_bua), 0)
FROM data_amba
WHERE experiment_name = 'matspr'
AND `year` = 2018
AND location_name = 'Stephen'
GROUP BY line_name;

Answer 2

可能是 MySQL WorkBench 的行为为整数 field.When 显示零，我运行相同SqlFiddle，它显示 null 如果 所有行都为 null 属于该类别。

MySQL Workbench, NULL 的平均值是 returning "0", 我需要它 return NULL

MySQL Workbench, average of NULL is returning "0", I need it to return NULL

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