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 3.3.4.6 Working with `NULL' Values
 ..................................
 
 The `NULL' value can be surprising until you get used to it.
 Conceptually, `NULL' means missing value or unknown value and it is
 treated somewhat differently than other values.  To test for `NULL',
 you cannot use the arithmetic comparison operators such as `=', `<', or
 `<>'.  To demonstrate this for yourself, try the following query:
 
      mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;
      +----------+-----------+----------+----------+
      | 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |
      +----------+-----------+----------+----------+
      |     NULL |      NULL |     NULL |     NULL |
      +----------+-----------+----------+----------+
 
 Clearly you get no meaningful results from these comparisons.  Use the
 `IS NULL' and `IS NOT NULL' operators instead:
 
      mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
      +-----------+---------------+
      | 1 IS NULL | 1 IS NOT NULL |
      +-----------+---------------+
      |         0 |             1 |
      +-----------+---------------+
 
 Note that in MySQL, `0' or `NULL' means false and anything else means
 true. The default truth value from a boolean operation is `1'.
 
 This special treatment of `NULL' is why, in the previous section, it
 was necessary to determine which animals are no longer alive using
 `death IS NOT NULL' instead of `death <> NULL'.
 
 Two `NULL' values are regarded as equal in a `GROUP BY'.
 
 When doing an `ORDER BY', `NULL' values are presented first if you do
 `ORDER BY ... ASC' and last if you do `ORDER BY ... DESC'.
 
 Note that MySQL 4.0.2 to 4.0.10 incorrectly always sorts `NULL' values
 first regardless of the sort direction.
 

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