void nvl(HplsqlParser.Expr_func_paramsContext ctx) {
    for (int i=0; i < ctx.func_param().size(); i++) {
      Var v = evalPop(ctx.func_param(i).expr());
      if (v.type != Var.Type.NULL) {
        exec.stackPush(v);
        return;
      }
    }
    evalNull();
  }
NVL function - Return first non-NULL expression
hplsql/src/main/java/org/apache/hive/hplsql/functions/FunctionMisc.java