I have a website that has an update query. Example.

Table name -> myTable
Table Content -> id(AI)
                 name  --> myName
                 age   --> 13
                 image --> (no values)
                 birthdate  --> (no values)

I only put values in name and age. In image and birthdate, I haven't put anything.
If I edit it and update it and I don't put anything in name and in age, I only put values in image and in birthdate. The output is like this. (This ouput I wanted)

Table name -> myTable
Table Content -> id(AI)
                 name  --> myName
                 age   --> 13
                 image --> myImage.jpg
                 birthdate  --> Feb. 31, 2010

This is my code:

<?php
  //mysql connect...
  //mysql select database...
$sql = "UPDATE myTable SET name = '".$name."', age = '".$age."', image = '".$image."', birthdate"'$birthdate.."' WHERE id = $id";
mysql_query($sql);
?>

I used this code but the output is:

Table name -> myTable
Table Content -> id(AI)
                 name  --> (no values)
                 age   --> (no values)
                 image --> myImage.jpg
                 birthdate  --> Feb. 31, 2010

Feel free to ask if you don't understand my question
Thank you