如何在mysql insert语句中放入PHP'not null'条件？

I have a page with two forms on it (and two submit buttons), and the forms link the page back to itself with action="" for the INSERT INTO statements. One of the INSERT INTO statements is:

<?php
$sql="INSERT INTO panelProduct (length_mm, width_mm, aperture)

VALUES ('$_POST[p_length_mm]','$_POST[p_width_mm]','$_POST[p_aperture]')";

if (!mysql_query($sql,$con))
  {
  die('Error: ' . mysql_error());
  }
echo "1 record added";
?>

One of the reasons I have put the form and the INSERT statements on the same page is that the form allows users to add products to an order, and there may be several products (so I thought the user could keep submitting until they're done...). The other reason was that I need to use a $_POST value sent from the previous page to do something else with the products they enter, and I don't know how to send it to more than one page.

Anyway, the problem I have found is that a new row is inserted into the database every time the page is refreshed, containing NULL values. This makes sense to me as the PHP will execute the statement above every time it encounters it. My question therefore is how I can make some sort of condition that will only execute the INSERT statement if the user enters something in the form and 'Submits'?

you could use the empty() method of PHP to check for null values in each of the variables.

Something like:

if(!empty($_POST['p_length_mm']) && !empty($_POST['p_width_mm']) && !empty($_POST['p_aperture']))
{
//execute your query here
}
else
{
//if there's an alternative you would like to happen if values are null, or just leave out.
}

Hope that does the trick.

You need a condition like this:

if ('POST' === $_SERVER['REQUEST_METHOD']) {
    // a form was posted
}

Of course, you still need to check whether your post actually contains the right stuff:

isset($_POST['p_length_mm'], $_POST['p_width_mm'], $_POST['p_aperture'])

Even better is to use filter_input_array() to sanitize your incoming data; your current code is open to SQL injection attacks. Using PDO or mysqli and prepared statements is the way to go.

You have to check if data come from your form. I'd suggest this approach - name your submit button (i.e. <input type="submit" name="my_submit_button" value="Whatever" /> and this condition to your insert code:

if( isset( $_POST['my_submit_button']) ) {
  .. process your post data
}

Also, always remember not to trust user provided data. Verify if all data expected are present in$_POST and format matches requirements (i.e. you ask for number and received letters etc). If all test passed, then you are ready to commit your data to DB.