Question: Is there any way to **clear** mysql **query cache** without restarting mySQL server?

Answer: 
I believe you can use...

```
RESET QUERY CACHE;

```

...if the user you're running as has reload rights. Alternatively, you can defragment the query cache via...

```
FLUSH QUERY CACHE;

```

See the [Query Cache Status and Maintenance](http://dev.mysql.com/doc/refman/5.0/en/query-cache-status-and-maintenance.html) section of the MySQL manual for more information.
In my system (Ubuntu 12.04) I found `RESET QUERY CACHE` and even restarting mysql server not enough. This was due to [memory disc caching](https://askubuntu.com/questions/155768/how-do-i-clean-or-disable-the-memory-cache).  

After each query, I clean the disc cache in the terminal: 

```
sync && echo 3 | sudo tee /proc/sys/vm/drop_caches

```

and then reset the query cache in mysql client:

```
RESET QUERY CACHE;

```