如何在 MySQL 中计算空值?
mysqlmysqli database
要在 MySQL 中计算空值，可以使用 CASE 语句。让我们首先看一个例子并创建一个表 −
mysql> create table DemoTable ( Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, FirstName varchar(20) ); Query OK, 0 rows affected (0.77 sec)
使用 insert 命令在表中插入一些记录 −
mysql> insert into DemoTable(FirstName) values('John');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable(FirstName) values(null);
Query OK, 1 row affected (0.25 sec)
mysql> insert into DemoTable(FirstName) values('');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable(FirstName) values('Larry');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable(FirstName) values('');
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable(FirstName) values(null);
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(FirstName) values(null);
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable(FirstName) values('Bob');
Query OK, 1 row affected (0.15 sec)
以下是使用 select 语句显示表中的所有记录的查询 −
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)
这是在 MySQL 中计算空值的查询 −
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)
