Show The Alert When No Data Inserted?

i have a form. i'm going to store the field into mysql. there are 2 conditions : one or all fields empty OR none is empty. when there's no empty field, the data will be stored. but, when there's empty field, no data will be stored and there will come up the alert "please fill all the fields!" here's the form :

<form method=POST enctype='multipart/form-data' onclick='show_alert()' action=insert.php>
<table>
<tr><td>Data 1</td><td><input type=text name='data1'> </td></tr>
<tr><td>Data 2</td><td><input type=text name='data2'> </td></tr>
<tr><td>Data 3</td><td><input type=text name='data3'> </td></tr>
<tr><td colspan='2'><input type=submit value='Save'><input type=button value=Cancel onclick=self.history.back()></td></tr>
</table></form>";
[Code]...

i don't know how to show the alert with javasript.