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;