mysql> CREATE TABLE linkedTable (
    ->         testTableId INT
    ->     ) ENGINE = INNODB;
Query OK, 0 rows affected (0.12 sec)

mysql> CREATE TABLE testTable (
    ->         id INT, 
    ->         data CHAR(50), 
    ->         timestamp TIMESTAMP(14)
    ->     ) ENGINE = INNODB;
Query OK, 0 rows affected, 1 warning (0.11 sec)

mysql> 
mysql> INSERT INTO linkedTable (testTableId)   VALUES (1);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO testTable   (id,data)       VALUES (1,'original data');
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM linkedTable LEFT JOIN testTable ON testTableId=id;
+-------------+------+---------------+---------------------+
| testTableId | id   | data          | timestamp           |
+-------------+------+---------------+---------------------+
|           1 |    1 | original data | 2009-09-19 11:33:40 | 
+-------------+------+---------------+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> # Do an update to testTable directly which does not change anything
mysql> UPDATE testTable SET data='original data' WHERE id=1;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

mysql> SELECT * FROM linkedTable LEFT JOIN testTable ON testTableId=id;
+-------------+------+---------------+---------------------+
| testTableId | id   | data          | timestamp           |
+-------------+------+---------------+---------------------+
|           1 |    1 | original data | 2009-09-19 11:33:40 | 
+-------------+------+---------------+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> # Protect the update with an additional condition in the WHERE clause
mysql> UPDATE linkedTable LEFT JOIN testTable ON testTableId=id SET data='original data' WHERE id=1 AND data<>'original data';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 0

mysql> SELECT * FROM linkedTable LEFT JOIN testTable ON testTableId=id;
+-------------+------+---------------+---------------------+
| testTableId | id   | data          | timestamp           |
+-------------+------+---------------+---------------------+
|           1 |    1 | original data | 2009-09-19 11:33:40 | 
+-------------+------+---------------+---------------------+
1 row in set (0.00 sec)

mysql> # Do an update to testTable linked to linkedTable which should not change anything
mysql> UPDATE linkedTable LEFT JOIN testTable ON testTableId=id SET data='original data' WHERE id=1;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

mysql> SELECT * FROM linkedTable LEFT JOIN testTable ON testTableId=id;
+-------------+------+---------------+---------------------+
| testTableId | id   | data          | timestamp           |
+-------------+------+---------------+---------------------+
|           1 |    1 | original data | 2009-09-19 11:33:40 | 
+-------------+------+---------------+---------------------+
1 row in set (0.00 sec)

mysql> # Do an update to testTable directly which does not change anything
mysql> UPDATE testTable SET data='original data' WHERE id=1;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

mysql> SELECT * FROM linkedTable LEFT JOIN testTable ON testTableId=id;
+-------------+------+---------------+---------------------+
| testTableId | id   | data          | timestamp           |
+-------------+------+---------------+---------------------+
|           1 |    1 | original data | 2009-09-19 11:33:40 | 
+-------------+------+---------------+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> # Protect the update with an additional condition in the WHERE clause
mysql> UPDATE linkedTable LEFT JOIN testTable ON testTableId=id SET data='original data' WHERE id=1 AND data<>'original data';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 0

mysql> SELECT * FROM linkedTable LEFT JOIN testTable ON testTableId=id;
+-------------+------+---------------+---------------------+
| testTableId | id   | data          | timestamp           |
+-------------+------+---------------+---------------------+
|           1 |    1 | original data | 2009-09-19 11:33:40 | 
+-------------+------+---------------+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> # Do an update to testTable linked to linkedTable which should not change anything
mysql> UPDATE linkedTable LEFT JOIN testTable ON testTableId=id SET data='original data' WHERE id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT * FROM linkedTable LEFT JOIN testTable ON testTableId=id;
+-------------+------+---------------+---------------------+
| testTableId | id   | data          | timestamp           |
+-------------+------+---------------+---------------------+
|           1 |    1 | original data | 2009-09-19 11:34:03 | 
+-------------+------+---------------+---------------------+
1 row in set (0.00 sec)