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
,
use the IS
NULL
and IS
NOT NULL
operators, as shown here:
mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
+-----------+---------------+
| 1 IS NULL | 1 IS NOT NULL |
+-----------+---------------+
| 0 | 1 |
+-----------+---------------+
You cannot use arithmetic comparison operators such as =
, <
,
or <>
to
test for NULL
.
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 |
+----------+-----------+----------+----------+
Because the result of any arithmetic comparison with NULL
is
also NULL
,
you cannot obtain any meaningful results from such comparisons.
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
.
轉自:
http://dev.mysql.com/doc/refman/5.0/en/working-with-null.html