Question

I’ve got a PHP function which receives some parameters. Each parameter corresponds to a VALUE of one SQL line.
What I would like to do, is to update the values according to the parameters ; but with having some parameters not mandatory. So if a parameter is set to ‘keep’, the VALUE has to keep its current value. This is where I’m stuck.

Here is the function :

function upt($id, $p1, $p2)
{
    if ($p1 === 'keep')
        $p1 = 'columnName1';
    if ($p2 === 'keep')
        $p2 = 'columnName2';
    $REQ = $DB->prepare('UPDATE ms_cart_products SET columnName1=:P1, columnName2=:P2 WHERE ID=:ID');
    $REQ->execute(array(
        ':P1' => $p1,
        ':P2' => $p2,
        ':ID' => $id));
    $REQ->closeCursor();
}

Obviously, this does not work (even if I had the secret hope that making $p1 = ‘columnName1’ would work as if I made COL1=COL1 directly in the SQL console).
The VALUE is literally set to ‘columnName’.

Any ideas ?

Thank you,
Florian

Answer 1

Let’s assume that you use either PostgreSQL or MySQL (I don’t mention other RDBMSes here, please tell us if you use something else). Then you can write an UPDATE like

...
SET columnName1 = COALESCE(NULLIF(:your_value, 'keep'), columnName1)
...

NULLIF returns its first value when the too values provided are different, otherwise it returns NULL. COALESCE then returns the first non-NULL value. This way, if you pass ‘keep’ to this expression, the inner NULLIF part evaluates to NULL, therefore the COALESCE returns the value kept in columnName1. Otherwise the NULLIF returns the passed value and COALESCE evaluates to that.

See the docs:

http://www.postgresql.org/docs/9.1/static/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL

or

http://dev.mysql.com/doc/refman/5.5/en/control-flow-functions.html#function_nullif

http://dev.mysql.com/doc/refman/5.5/en/comparison-operators.html#function_coalesce