trouble with SQL Update

I'm trying to update a mastertable when records in child are being changed. I've added this code at Events / Add page / After record added:

[codebox]global $conn;

$strUpdate = "UPDATE mastertable SET masterfield_1=".$values["childfield_1"]." WHERE id=".$values["id"];

echo $strUpdate;

db_exec($strUpdate,$conn);[/codebox]
The code seems to be alright since I don't get an error message but still the update doesn't work. Also I've experimented with INSERT INTO - no insert either??!!?? Here's what I've tried:
[codebox]global $conn,$strTableName;
$strSQLSave = "INSERT INTO mastertable WHERE id=".$values["id"]."(masterfield_1, masterfield_2) values ('";
$strSQLSave .= $values["childfield_1"]."','";

$strSQLSave .= $values["childfield_2"];
$strSQLSave .= "')";

db_exec($strSQLSave,$conn);[/codebox]
hope you guys can help me on that, it's driving me nuts and I urgently need this!! thanks for helping!!