NULL is valuable, it has its own representation in MYSQL, and the result of logical operation is NULL, such as 2 and NULL.

However, a null value means that there is no such item, which is not considered in the operation.

So when PHP is read, it is different. NULL I don't know what the value is (I forgot), and an empty string will show that there is no such item (it is empty, otherwise it will make an error, I forgot) ` ~

So when programming, you should pay attention to what is stored in your database. If you remember, you shouldn't encounter this problem again.