Similar SQLs with session variable produce different result - mysql

Question

MariaDB 10.1.18
Table P (id int AUTO_INCREMENT, rownum int, code int, s int, PRIMARY KEY(id));
select id, rownum, code, s from P order by id;
+----+--------+------+------+
| id | rownum | code | s |
+----+--------+------+------+
| 1 | 1 | 5 | 1 |
| 2 | 2 | 5 | 1 |
| 3 | 3 | 5 | 1 |
| 4 | 4 | 5 | 1 |
| 5 | 5 | 5 | 1 |
| 6 | 6 | 7 | 1 |
| 7 | 7 | 7 | 1 |
| 8 | 8 | 7 | 1 |
| 9 | 9 | 7 | 1 |
| 10 | 10 | 7 | 1 |
+----+--------+------+------+
Issue: the following 2 queries are very similar: 1st query join on id, 2nd join on rownum; the id and rownum columns are having the same values (see table above), but the query result is different in calculated column N:
Query 1: join on id column
SELECT P.id, P.rownum, P2.s,
IF(P2.s IS NULL, #val:=#val+1, #val) as N
FROM P CROSS JOIN (SELECT #val:=0) init
LEFT JOIN P P2
ON (P.id+1=P2.id AND P.s=1 AND P2.s=1 AND P.code = P2.code)
ORDER BY P.id;
+----+--------+------+------+
| id | rownum | s | N |
+----+--------+------+------+
| 1 | 1 | 1 | 0 |
| 2 | 2 | 1 | 0 |
| 3 | 3 | 1 | 0 |
| 4 | 4 | 1 | 0 |
| 5 | 5 | NULL | 1 |
| 6 | 6 | 1 | 1 |
| 7 | 7 | 1 | 1 |
| 8 | 8 | 1 | 1 |
| 9 | 9 | 1 | 1 |
| 10 | 10 | NULL | 2 |
+----+--------+------+------+
Query 2: join on rownum column
SELECT P.id, P.rownum, P2.s,
IF(P2.s IS NULL, #val:=#val+1, #val) as N
FROM P CROSS JOIN (SELECT #val:=0) init
LEFT JOIN P P2
ON (P.rownum+1=P2.rownum AND P.s=1 AND P2.s=1 AND P.code = P2.code)
ORDER BY P.id;
+----+--------+------+------+
| id | rownum | s | N |
+----+--------+------+------+
| 1 | 1 | 1 | 0 |
| 2 | 2 | 1 | 0 |
| 3 | 3 | 1 | 0 |
| 4 | 4 | 1 | 0 |
| 5 | 5 | NULL | 1 |
| 6 | 6 | 1 | 0 |
| 7 | 7 | 1 | 0 |
| 8 | 8 | 1 | 0 |
| 9 | 9 | 1 | 0 |
| 10 | 10 | NULL | 2 |
+----+--------+------+------+

Answer 1

As explicitly documented both in MariaDB knowledge base and MySQL manual, you should not read a user-defined variable and set its value in the same statement, unless this statement is SET. For other statements it is unsafe and the result is not guaranteed, as your example clearly demonstrates.
Additional note: variables you are talking about in your question are more commonly referred to as 'user-defined variables' or 'user variables', as opposed to 'system variables', 'global variables' and 'session variables', each of which imply system-defined variables, either all of them or limited to the given scope.