int c = a;
a = b;
b = c;

##</code><code>##

a = a^b;
b = a^b;
a = a^b;

##</code><code>##

of a=1 and b=0. 
a = a^b = 1 xor 0 = 1
b = a^b = 1 xor 0 = 1
a = a^b = 1 xor 1 = 0

##</code><code>##

mysql> select * from test;
+------+------+
| id   | num  |
+------+------+
|    1 |  100 |
|    2 |  200 |
+------+------+
2 rows in set (0.00 sec)

mysql> update test set id=666 where id=1; update test set id=1 where id=2; update test set id=2 where id=666;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from test;
+------+------+
| id   | num  |
+------+------+
|    2 |  100 |
|    1 |  200 |
+------+------+
2 rows in set (0.00 sec)

mysql>