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What's your approach of updating rows coming from a checkbox?

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Newbie question guys..


Just want to know the best approach on this before I code it.


I have this merge table called "item_category" an item could have multiple rows base on how many categories the user chose.


I have this on my mind...


Delete all the item's rows(using item_ID PK)

and then insert it again with the new one.



So there's no update sql that will happen, only delete and insert...


What do you think guys?

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I usually avoid the DELETE and INSERT approach for several reasons, most of which are just personal preference. If I have a group of checkboxes, such as categories, I name them so it produces an array of the category IDs in the POST array: <INPUT type="checkbox" name="chkCategory[]" value="$CategoryID" ... then I use the following approach -- remember, un-checked checkboxes will not be posted:


$selectedIDs = $_POST['chkCategory'];
# Do the sanitation (make sure they are integers)

# Delete any existing entries that are not now selected
$query = "DELETE FROM ItemCategories WHERE ItemID = $itemID AND CategoryID NOT IN (" . implode(',', $selectedIDs) . ")";

# Insert any selected entries that are not already there
$query = "INSERT INTO ItemCategories (ItemID, CategoryID)
SELECT $itemID, CategoryID FROM Categories 
WHERE CategoryID IN (" . implode(',', $selectedIDs) . ")
AND NOT EXISTS (SELECT * FROM ItemCategories WHERE ItemID = $itemID AND CategoryID IN (" . implode(',', $selectedIDs) . ") )";

The subquery is necessary since mySql will not allow you to JOIN to the table you are inserting.


Note: You can avoid the subquery if you have a UNIQUE INDEX on (in this case) ItemID,CategoryID by using INSERT IGNORE


My reasons for this approach (in no particular order):

  • Avoids unnecessary holes in the AUTO INCREMENT value in the table (if any);
  • Avoids "wasting" AUTO INCREMENT values (if any) so I don't have to worry about running out;
  • Does not fragment the table and indexes as bad as DELETE and INSERT;
  • If I have a CreateTime column (and I usually do), we are not loosing that data;


Disclaimer: I'm not 100% sure of that syntax, I'm not at home where I can check my code base. But I think you can see the logic I use.

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Thanks for the suggestion DavidAM


I'll definitely look into that.  8)

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I'm really interested in the reason why you don't want to run an UPDATE query on the database? That is the real question, at least in my mind. ;)


Depending upon your reasoning for this decision the answer you got is either the correct one, or this whole thread is a red herring. Which is why I'm wondering.

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