MySQL SUM()函数如何计算带有NULL值的列

假设我们正在计算一列值的总和，该列还包含NULL值，则MySQL的SUM()函数将忽略NULL值并对其余值进行求和。要了解这一点，请考虑以下示例“employee”表的详细信息：

mysql> Select * from Employee;
+----+--------+--------+
| ID | Name   | Salary |
+----+--------+--------+
| 1  | Gaurav | 50000  |
| 2  | Rahul  | 20000  |
| 3  | Advik  | 25000  |
| 4  | Aarav  | 65000  |
| 5  | Ram    | 20000  |
| 6  | Mohan  | 30000  |
| 7  | Aryan  | NULL   |
| 8  | Vinay  | NULL   |
+----+--------+--------+
8 rows in set (0.00 sec)

现在，假设我们要计算上表中员工的总工资，则在使用SUM()函数进行计算时，它会忽略NULL值。以下查询将生成所需的结果集。

mysql> Select SUM(Salary) from Employee;
+-------------+
| SUM(Salary) |
+-------------+
| 210000      |
+-------------+
1 row in set (0.00 sec)

可以使用以下查询来验证这一点。

mysql> Select SUM(Salary) from Employee WHERE Salary IS NOT NULL;
+-------------+
| SUM(Salary) |
+-------------+
|   210000    |
+-------------+
1 row in set (0.00 sec)

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