SQL Where not equal does not return NULL results

As per your where clause it compares null != 'Iron', which evaluates to UNKNOWN which neither true nor false based on SQL's 3 way logic. Hence it is not returned by the query.

If you need the null row to be resulted, you should use

where content !='Iron' or content is null

Edit: One more option is to use the relational null-safe equality operator <=> with a negation.

not content <=> 'Iron'

From the documentation,

NULL-safe equal. This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL.

The <=> operator is equivalent to the standard SQL IS NOT DISTINCT FROM operator.

mysql> 
   SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL
   UNION
   SELECT 1 = 1, NULL = NULL, 1 = NULL;
   

Returns

  1 vs 1  |  NULL vs NULL  | 1 vs NULL |  Comment
  --------------------------------------------------
     1    |        1       |    0      |  for <=>
     1    |       NULL     |   NULL    |  for  =

ISNULL(content, '') = 'Iron'

This will convert the null to an empty string.