sql - Why does this simple MySQL query not return the row? -

i have row in table users username test. reason, though, query returns empty result set.

select `id` `users` `username` = "test" , `id` != null; 

however, if remove `id` != null segment, query returns result id = 1.

but 1 != null. how happening?

the id field non-nullable , auto-increment.

thanks!

the query doesn't return row because predicate " id != null " never return true.

th reason boolean logic in sql 3 valued. boolean can have values of true, false or null.

and inequality comparison return null whenever 1 (or both) of values being compared null.

the sql standard means compare null use id null or id not null. mysql adds convenient null-safe comparison operator return true or false:

col <=> null. or, in case not (col <=> null)