Question

table person:

name
type enum('admin','user','random') NULL DEFAULT NULL

When this query is performed, it doesnt return the records that have a type of NULL

select * from person where type != 'admin';

Answer 1

null cannot be tested for with (in)equality statements. you need to use IS NULL. e.g.

select *
from person
where (type != 'admin') or (type IS NULL)

e.g. null is “contagious”

null > x    -> null
null = x    -> null
null = null -> null
null + 1    -> null
null * 1    -> null

etc… it’s basically “unknown”. Mixing known and unknown in sql makes the result unknown, always. Hence the special ifnull(), coalesce(), and “if null” tests/functions.