MySQL 如何使用单个MySQL查询计算列值并忽略NULL

为此，您可以使用不包括NULL值的COUNT()方法。首先，让我们创建一个表 –

mysql> create table DemoTable
   -> (
   -> Name varchar(100),
   -> CountryName varchar(100)
   -> );
Query OK, 0 rows affected (0.49 sec)

使用insert命令在表中插入一些记录 –

mysql> insert into DemoTable values('John',null);
Query OK, 1 row affected (0.17 sec)

mysql> insert into DemoTable values('Chris','US');
Query OK, 1 row affected (0.12 sec)

mysql> insert into DemoTable values('Robert',null);
Query OK, 1 row affected (0.19 sec)

mysql> insert into DemoTable values('Bob','UK');
Query OK, 1 row affected (0.57 sec)

使用select语句显示表中的所有记录 –

mysql> select *from DemoTable;

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输出

这将产生以下输出结果 –

+--------+-------------+
| Name   | CountryName |
+--------+-------------+
| John   | NULL        |
| Chris  | US          |
| Robert | NULL        |
| Bob    | UK          |
+--------+-------------+
4 rows in set (0.00 sec)

这是使用COUNT()和忽略NULL的查询 –

mysql> select count(Name) AS TotalName,count(CountryName) AS CountryWhichIsNotNull from DemoTable;

输出

这将产生以下输出结果 –

+-----------+-----------------------+
| TotalName | CountryWhichIsNotNull |
+-----------+-----------------------+
| 4         | 2                     |
+-----------+-----------------------+
1 row in set (0.00 sec)