mysql 列值中（NULL）和空之间的区别

Question

I am working in a company where we use Spring -Hibernate and mysql database at backend.

There is a table tc_wallet
In this table i have an column tips_type which has values

1. Collection,Payable
2. '' (empty)
3. NULL --> No value has been initialized

Now when i fire a query:

SELECT * 
FROM `tc_wallet` 
WHERE 
    login_id = 'gaurav.wakharkar' 
    AND `delete_flag` = 'F' 
    AND `tips_type` != 'Collection' 

I get results which has column value as '' (empty).

Login_id            tips_type   
gaurav.wakharkar                 
gaurav.wakharkar         
gaurav.wakharkar   

But even (NULL) is != 'Collection' should satisfy the above condition.
So according to me the result should have been .

Login_id            tips_type   
gaurav.wakharkar                 
gaurav.wakharkar         
gaurav.wakharkar       
gaurav.wakharkar    (NULL)   
gaurav.wakharkar    (NULL)

Is there some issue while checking/comparing values with (NULL) ?
Does it behave differently ?

Answer 1

To check for nullness, you want to use IS NULL . Comparing NULL to something else with the equality operator (or the inequality operator) is always false.

Consider:

SELECT * 
FROM `tc_wallet` 
WHERE 
    login_id = 'gaurav.wakharkar' 
    AND `delete_flag` = 'F' 
    AND (`tips_type` IS NULL OR `tips_type` != 'Collection')

Answer 2

change your query to

SELECT 
  * 
FROM
  `tc_wallet` 
WHERE login_id = 'gaurav.wakharkar' 
  AND `delete_flag` = 'F' 
  AND (`tips_type` != 'Collection' or `tips_type` is null)