code: func builtinIfNull(args []types.Datum, _ context.Context) (d types.Datum, err error) {
  	// ifnull(expr1, expr2)
  	// if expr1 is not null, return expr1, otherwise, return expr2
  	v1 := args[0]
  	v2 := args[1]
  
  	if !v1.IsNull() {
  		return v1, nil
  	}
  
  	return v2, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/control-flow-functions.html#function_ifnull