当我们需要知道mysql中的某个字段是否为空？怎么实现呢？
这里提供了5种方法进行查询：
isnull()
select * from users where email = 'xxxx' and isnull(deletedAt)
is null
select * from users where email = 'xxxx' and deletedAt is null
is not null
select * from users where email = 'xxxx' and deletedAt is not null
!isnull()
select * from users where email = 'xxxx' and !isnull(deletedAt) select * from users where email = 'xxxx' and not isnull(deletedAt)
isfull
当查询条件为 null，用指定字符替代
select name, isfull(gender,'未知') as gender from users where email = 'xxxx'下载本文
