SQL where not equal 不返回 NULL 结果

Question

table 'materials'
id   sku  content
10   IT2   Iron
11   IT3   Steel
12   IT4   Steel
13   IT5   NULL
14   IT6   Iron
15   IT7   Glass

select id, sku, content from materials where content !='Iron';

Returns Result:

id   sku   content
11   IT3   Steel
12   IT4   Steel
15   IT7   Glass

Why is id #13 with the NULL value not returned in the result set? Using MYSQL.

Answer 1

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  =

Answer 2

ISNULL(content, '') = 'Iron'

这会将 null 转换为空字符串。