mysql table join and repeating region of checkboxes

Question

I have two tables I need to join and need to check a html check box of the repeating region if the contact_id field is in the list.

Table one "contact_to_category"- id, contact_id and category_id

Table two "category" - category_id, category, status

I am trying to create an update page for the contact and show all categories from table two and check the checkbox if the clients contact_id is in table one. This way the person editing can see what categories are already active.

Here is what I have tried but cannot figure out how to check the checkboxes if their id is listed in table one? I have tried joins but don't have a variable to say if a=b then echo "checked" in the repeating region of checkboxes.

Thanks!

 $query_flags = "SELECT a.category, a.status, a.category_id FROM category a LEFT JOIN contact_to_category ON contact_to_category.category_id = a.category_id  AND contact_to_category.contact_id = '$contactid'  AND a.status = 1";

        <?php do { ?>
          <table width="327" border="0" cellspacing="0" cellpadding="1">
            <tr>
              <td width="20" align="right"><input name="catlist[]" type="checkbox" id="catlist[]" value="<?php echo $row_flags['category_id']; ?>" />
              <label for="catlist[]"></label></td>
              <td width="226"><?php echo $row_flags['category']; ?></td>
            </tr>
          </table>
          <?php } while ($row_flags = mysql_fetch_assoc($flags)); ?>

Answer 1

You are doing a LEFT JOIN and when the category_id is not in the first table there will be null row so this is how you can checked your check boxes i have added one column from the first table contact_to_category.category_id AS second_cat_id to check whether it exists or not

 $query_flags = "SELECT a.category, a.status, a.category_id 
,contact_to_category.category_id AS second_cat_id FROM category a 
LEFT JOIN contact_to_category ON contact_to_category.category_id = a.category_id 
 AND contact_to_category.contact_id = '$contactid'  AND a.status = 1";
$row_flags=mysql_query($query_flags); i assume you have done this in your code
        <?php do { ?>
          <table width="327" border="0" cellspacing="0" cellpadding="1">
            <tr>
              <td width="20" align="right">
   <?php if(!empty($row_flags['second_cat_id '])){ ?>

 <input name="catlist[]" type="checkbox" checked="checked" id="catlist[]" value="<?php echo $row_flags['category_id']; ?>" />
   <?php } else{?>
 <input name="catlist[]" type="checkbox" id="catlist[]" value="<?php echo $row_flags['category_id']; ?>" />
   <?php } ?>
              <label for="catlist[]"></label></td>
              <td width="226"><?php echo $row_flags['category']; ?></td>
            </tr>
          </table>
          <?php } while ($row_flags = mysql_fetch_assoc($flags)); ?>