How do uninitialized variables differ from NULL in Mysql?
There are two ways to set a variable from a query, and they behave differently, especially when no record is matched.

You can SELECT ... INTO the variable...

select first_name into @z from actor where actor_id = 999999;


Or you can SET it to the value returned by a scalar subquery.

set @z = (select first_name from actor where actor_id = 999999);


Illustrating the difference with the sample database...

mysql&gt; use sakila;
Database changed


Our variable starts out NULL.

mysql&gt; select @z;
+------+
| @z   |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

mysql&gt; SELECT first_name INTO @z FROM actor WHERE actor_id = 1;
Query OK, 1 row affected (0.00 sec)


We matched a row and our variable is set.

mysql&gt; select @z;
+----------+
| @z       |
+----------+
| PENELOPE |
+----------+
1 row in set (0.00 sec)


We try to select a record that doesn't exist:

mysql&gt; select first_name into @z from actor where actor_id = 999999;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql&gt; show warnings;
+---------+------+-----------------------------------------------------+
| Level   | Code | Message                                             |
+---------+------+-----------------------------------------------------+
| Warning | 1329 | No data - zero rows fetched, selected, or processed |
+---------+------+-----------------------------------------------------+
1 row in set (0.00 sec)


So what's in our variable now?

mysql&gt; select @z;
+----------+
| @z       |
+----------+
| PENELOPE |
+----------+
1 row in set (0.00 sec)


I think this explains the confusion.  When no row is found, the variable is unchanged from its previous value... so it's not that you have to "set it to null before you use it," it's that you have to reset it to null before you use it again, if you are doing something like SELECT ... INTO that won't reset the value if nothing is found.

On the other hand, if we use the other construct, the variable is reset when no record is matched.

mysql&gt; select @z;
+----------+
| @z       |
+----------+
| PENELOPE |
+----------+
1 row in set (0.00 sec)

mysql&gt; set @z = (select first_name from actor where actor_id = 999999);
Query OK, 0 rows affected (0.00 sec)

mysql&gt; select @z;
+------+
| @z   |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

mysql&gt;