Working with NULL Values

Working with NULL Values
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 [1188] and IS NOT NULL [1188] 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 = [1186], [1187] 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 |
+----------+-----------+----------+----------+
| 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 |
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