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如何在MySQL中计算空值？

2024-04-28 06:49:04 419

要在MySQL中计算空值，可以使用CASE语句。我们首先来看一个示例并创建一个表-

mysql> create table DemoTable
(
   Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   FirstName varchar(20)
);

使用插入命令在表中插入一些记录-

mysql> insert into DemoTable(FirstName) values('John');
mysql> insert into DemoTable(FirstName) values(null);
mysql> insert into DemoTable(FirstName) values('');
mysql> insert into DemoTable(FirstName) values('Larry');
mysql> insert into DemoTable(FirstName) values('');
mysql> insert into DemoTable(FirstName) values(null);
mysql> insert into DemoTable(FirstName) values(null);
mysql> insert into DemoTable(FirstName) values('Bob');

以下是使用select语句显示表中所有记录的查询-

mysql> select *from DemoTable;

这将产生以下输出-

+----+-----------+
| Id | FirstName |
+----+-----------+
| 1  | John      |
| 2  | NULL      |
| 3  |           |
| 4  | Larry     |
| 5  |           |
| 6  | NULL      |
| 7  | NULL      |
| 8  | Bob       |
+----+-----------+
8 rows in set (0.00 sec)

这是对MySQL中的Null值进行计数的查询-

mysql> select sum(case when FirstName IS NULL then 1 else 0 end) as NUMBER_OF_NULL_VALUE from DemoTable;

这将产生以下输出-

+----------------------+
| NUMBER_OF_NULL_VALUE |
+----------------------+
| 3                    |
+----------------------+
1 row in set (0.00 sec)

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如何在MySQL中计算空值？

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