code: func builtinConcatWS(args []types.Datum, _ context.Context) (d types.Datum, err error) {
  	var sep string
  	s := make([]string, 0, len(args))
  	for i, a := range args {
  		if a.IsNull() {
  			if i == 0 {
  				return d, nil
  			}
  			continue
  		}
  		ss, err := a.ToString()
  		if err != nil {
  			return d, errors.Trace(err)
  		}
  
  		if i == 0 {
  			sep = ss
  			continue
  		}
  		s = append(s, ss)
  	}
  
  	d.SetString(strings.Join(s, sep))
  	return d, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_concat-ws