Display Validations using Dynamic Actions in Oracle APEX

Button --> Dynamic Actions --> Event: Click --> True --> Execute Javascript Code

NOT NULL Check
*****************************************

if ($v('P7_CITYNAME')=== undefined || $v('P7_CITYNAME')== null || $v('P7_CITYNAME').length <= 0)
 {
    apex.message.alert('Please Enter City Name.');
}
if ($v('P7_COUNTRYNAME')=== undefined || $v('P7_COUNTRYNAME')== null || $v('P7_COUNTRYNAME').length <= 0)
 {
    apex.message.alert('Please Enter Country Name.');
}

or


if ($v('P7_CITYNAME') == "")
{
alert("Please Enter City Name.");
}
if ($v('P7_COUNTRYNAME') == "")
{
alert("Please Enter Country Name.");
}

Duplicate check
*************************************************
Execute PL/SQL Code

DECLARE
   v_dup   NUMBER (1) := 0;
BEGIN
   SELECT COUNT (*)
     INTO v_dup
     FROM CITIES
    WHERE CITYNAME = :P7_CITYNAME;

   IF v_dup > 0
   THEN
      :P7_CITYDUP := 'Y';
   ELSE
      :P7_CITYDUP := 'N';
   END IF;
END;


Items to Submit: P7_CITYNAME
Items to Return: P7_CITYNAME

Execute Javascript Code

var nameExists = $v('P7_CITYDUP') === 'Y';
var cityNull = $v('P7_CITYNAME') === '';

if (nameExists) {
  if (!cityNull) {
  alert('This City Name already exists. Please enter a different City Name');
}
}