如何使用数组更新

I am looking for some inspiration from someone wiser than me with PHP/MySQL.

In have a database application, and in this instance there are two primary tables and one child table.

Primary Table 1 - Documents Primary Table 2 - JobDesriptions Child Table - LnkDocuments_JobDescriptions, which as the title suggests is a one to many relational table between the Document and Job Description Table. In my Documents Table I have a field which is a lookup of JobDescriptions and presents options as a checkbox, this field is called 'AppliesTo', because of the way the application works, the field stores the results as a string, eg "1,2,3,4,5) I have used the explode function to turn this into an array and then insert each record into the child table, as I prefer 1-2-many relationships. This is the code that I have, and it works.

$jdarray = explode(',',$values['AppliesTo']);

foreach($jdarray as $item)
{
$sql2 = "INSERT INTO LnkDocuments_JobDescriptions (DocumentFk, JobDescriptionFk) 
values ('".$keys["DocumentPk"]."', '$item')"; CustomQuery($sql2);
}

The problem I now have is that if that table gets updated, I need to also update the child table, i have tried this code (but quickly realised that it is wrong):

$jdarray = explode(',',$values['AppliesTo']);

foreach($jdarray as $item)
{
    $sql = "SELECT * FROM LnkDocuments_JobDescriptions WHERE DocumentFk='".$keys["DocumentPk"]."' AND JobDescriptionFk='$item'"; ;
    $num_rows = mysql_num_rows(CustomQuery($sql));

    if ($num_rows > 0) //Delete Record
        {
            $sql2 = "DELETE FROM LnkDocuments_JobDescriptions WHERE DocumentFk='".$keys["DocumentPk"]."' AND JobDescriptionFk='$item'"; CustomQuery($sql2);
            echo "JD FK : ".$item." deleted";
        }
    else  //Insert Record
        {
            $sql3 = "INSERT INTO LnkDocuments_JobDescriptions (DocumentFk, JobDescriptionFk) 
            values ('".$keys["DocumentPk"]."', '$item')"; CustomQuery($sql3);   
            echo "JD FK : ".$item." added";
        }

}

It occured to me that I need to compare differences in the arrays, but havent got a clue how to do this, but this is what I need:

If I can get $oldarray and $new array to compare, for example

if in old array there were values 1,2,3,4 and in $newarray there were values 1,2,3,5, I want the code to loop through each value to determine if there is a change, e.g. if value exists in old and new array then do nothing, if value exists in old array but not new then delete, if value exists in new array but not old then insert.

I have also thought about just deleting all associated records and adding again, but think this is bad practice and will result in high number primary key, also it is worth noting that in my example there are only 5 options, this is just for testing, in reality there could be dozens.

Thanks in advance

If you are trying to optimize things I'm not sure that reading the values already present in the table and then deleting only those are not in the new version while inserting the missing records is the best way to go. In my opinion it would be much faster to just delete everything in one query, then insert all records in one query. Try something like this:

$item_list = implode( ',' , $jdarray );
$delete_query = "DELETE FROM LnkDocuments_JobDescriptions WHERE DocumentFk='".$keys["DocumentPk"]."' AND JobDescriptionFk IN ( $item_list )";
CustomQuery($delete_query);

$document_key = "'" . $keys["DocumentPk"] . "'";
$item_list_to_insert = "($document_key, " . implode( "), ($document_key, ", $jdarray ) . ")";

$insert_query = "INSERT INTO LnkDocuments_JobDescriptions (DocumentFk, JobDescriptionFk) VALUES " . $item_list_to_insert;
CustomQuery($insert_query);

Note: I didn't test this, there might some debugging needed.