mysql count distinct null 使用注意事项

1 用一个例子来讲解一个问题，现在又一个库表hello,表内容如下：

id name

1 Null
2 Null
3 Null
4 Null
5 Null

hello表一共两个字段：id和name，name is null。

第一条SQL：SELECT COUNT(id) FROM hello; 查询结果：5，正确。

第二条SQL：SELECT COUNT(*) FROM hello; 查询结果：5，正确。

第三条SQL：SELECT COUNT(name) FROM hello; 查询结果：0，错误。

第四条SQL：SELECT COUNT(DISTINCT id,name) FROM hello;查询结果：0，错误。

2 第二条SQL和第三条SQL查询错误的原因：

2.1 COUNT(),MIN(), and SUM() ignore NULL values.

2.2 The exception to this is COUNT(*), which counts rows and not individual column values.

2.3 For example, the following statement produces two counts. The first is a count of the number of rows in the table, and the second is a count of the number of non-NULL values in the age column:

mysql> SELECT COUNT(*), COUNT(age) FROM person;