Sorting and NULL Values
Problem
You want to sort a column that may
contain NULL
values.
Solution
The placement of NULL
values in a sorted list has
changed over time and depends on your version of MySQL. If
NULL
values don’t come out in the
desired position within the sort order, trick them into appearing
where you want.
Discussion
When a sorted column contains NULL
values, MySQL
puts them all together in the sort order. It may seem a bit odd that
NULL
values are grouped this way, given that (as
the following query shows) they are not considered equal in
comparisons:
mysql> SELECT NULL = NULL;
+-------------+
| NULL = NULL |
+-------------+
| NULL |
+-------------+
On the other hand, NULL
values conceptually do
seem more similar to each other than to non-NULL
values, and there’s no good way to distinguish one
NULL
from another, anyway. However, although
NULL
values group together, they may be placed at
the beginning or end of the sort order, depending on your version of
MySQL. Prior to MySQL 4.0.2, NULL
values sort to
the beginning of the order (or at the end, if you specify
DESC
). From 4.0.2 on, MySQL sorts
NULL
values according to the ANSI SQL
specification, and thus always places them first in the sort order,
regardless of whether or not you specify DESC
.
Despite these differences, if you want NULL
values
at one end or the other of the sort order, you can force them to be
placed where you want no matter which version of MySQL
you’re using. Suppose you have a table
t
with the following ...
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