解决从集合运算到mysql的not like找不出NULL的问题

#查询语句1
#找出表中id中含有‘A'或‘B'或‘C'的字段
select * from table1 where id like '%A%' or id like '%B%' or id like '%C%' ;
#成功查出300条

#查询语句2
#找出表中id中不含有‘A'且不含有‘B'且不含有‘C'的字段
select * from table1 where id not like '%A%' and id not like '%B%' and id not like '%C%' ;
#成功查出400条

#查询语句3
#找出表中id中不含有‘A'或‘B'或‘C'的字段
select * from table1 where id not in (select * from table1 where id like '%A%' or id like '%B%' or id like '%C%' );
#成功查出700条

select * from user
where username not in 
(
select username 
from user
where id != 2
)

select * from user
where username not in 
(
select username 
from user
where id != 2 and username is not null
)