struct _externalOptions {
    int id;
    string value;
}

/*------------------------------------------------------------------------------
Get options from external database
------------------------------------------------------------------------------*/

_externalOptions[] exOptions;
exOptions = sql("myDatabase", "SELECT id, value FROM myTable");

/*------------------------------------------------------------------------------
Get the curren list of options from the custom field and compare with new data
------------------------------------------------------------------------------*/

//prepare external data for comparison
string[] exLazyCheck;
for(_externalOptions ex in exOptions) {
    exLazyCheck[ex.value] = ex.value;
}

//get and prepare internal data for comparison
JCustomFieldOption[] customFieldOptions = admGetCustomFieldOptions("My Select List", {});
JCustomFieldOption[] customFieldOptionsMapped;
for(JCustomFieldOption option in customFieldOptions) {
    customFieldOptionsMapped[option.value] = option;
}

string[] intLazyCheck;
for(JCustomFieldOption option in customFieldOptions) {
    intLazyCheck[option.value] = option.value;
}

//compare the data
string[] newOptions = arrayDiff(exLazyCheck, intLazyCheck);
string[] oldOptions = arrayDiff(intLazyCheck, exLazyCheck);

/*------------------------------------------------------------------------------
Compare external options to see which options are new and need to be added
------------------------------------------------------------------------------*/

JCustomFieldOption[] addOptions;
for(string new in newOptions) {
    JCustomFieldOption option;
    option.value = new;
    option.disabled = false;
    
    addOptions += option;
}

//commit changes
admAddCustomFieldOptions("My Select List", addOptions);

/*------------------------------------------------------------------------------
Compare external options to see which options need to be removed
------------------------------------------------------------------------------*/

string[] removeOptions;
for(string old in oldOptions) {
    JCustomFieldOption option;
    option = customFieldOptionsMapped[old];
    
    removeOptions += option.id;
}

//commit changes
admDeleteCustomFieldOptions("My Select List", removeOptions, {});