在mysql中的不等于，<>，!=，is not
多说无益，来个实例！！！
一个简单地表数据：
select * from user where address != "北京"
select * from user where address <> "北京"
select * from user where address = null
select * from user where address is null
select * from user where address != null
总结：
select * from user where address != "北京"
select * from user where address <> "北京"
select * from user where address = null
select * from user where address is null
select * from user where address != null
select * from user where address is not null短短几条语句，三个极其常见的点，或许我们在回答的时候却不知所措，犹豫不决。
在<>和！=是等价的。在某字段不等于某值（非空的值）时，输出的结果此字段为空不输出。
is 和 is not 用于和 null 结合，我称它为不是，不是空