建表语句
create table user (id int, name varchar(20), index(id))engine=innodb;
insert into user values(1,'阳光女孩');
建表时请手动添加一行id为null 的数据
1.负向查询索引不能命中，导致全表扫描
explain select * from user where id != 2;
type：all
2. 不等于（!=）不等同于 is null
select * from user where id != 2; 不会查询去id为null 的数据
3.此时使用关键字or也会导致全表扫描，建议使用union进行优化
select * from user where id is null 走索引
select * from user where id = 1 走索引
select * from user where id = 1 or id is null 不走索引
这时建议使用union优化：
select * from user where id = 1 union select * from user where id = 1 or id is null 走索引
