The NULL value can be surprising until you
get used to it. Conceptually, NULL means
“a missing unknown value” and it is treated
somewhat differently from 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 |
+-----------+---------------+
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.
A common error when working with NULL is to
assume that it is not possible to insert a zero or an empty
string into a column defined as NOT NULL,
but this is not the case. These are in fact values, whereas
NULL means “not having a
value.” You can test this easily enough by using
IS [NOT] NULL as shown:
mysql> SELECT 0 IS NULL, 0 IS NOT NULL, '' IS NULL, '' IS NOT NULL;
+-----------+---------------+------------+----------------+
| 0 IS NULL | 0 IS NOT NULL | '' IS NULL | '' IS NOT NULL |
+-----------+---------------+------------+----------------+
| 0 | 1 | 0 | 1 |
+-----------+---------------+------------+----------------+
Thus it is entirely possible to insert a zero or empty string
into a NOT NULL column, as these are in
fact NOT NULL. See
Section C.5.5.3, “Problems with NULL Values”.