写一个golang函数，入参是一个slice和数据库表名，slice为struct类型，字段有tag叫db，这个函数的功能是根据入参拼接批量update sql。sql语句用case when，不要用循环

func GenerateBatchUpdateSQL(data interface{}, tableName string) (string, error) {
    v := reflect.ValueOf(data)
    if v.Kind() != reflect.Slice {
        return "", errors.New("input data is not a slice")
    }

    if v.Len() == 0 {
        return "", errors.New("input slice is empty")
    }

    t := v.Index(0).Type()

    var fields, values string
    for i := 0; i < t.NumField(); i++ {
        tag := t.Field(i).Tag.Get("db")
        if tag != "" {
            fields += tag + ","
            values += "WHEN ? THEN " + tag + " "
        }
    }

    fields = strings.TrimRight(fields, ",")
    values = strings.TrimSpace(values)

    sql := fmt.Sprintf("UPDATE %s SET ", tableName) + fields + " = CASE id " + values + " END"

    return sql, nil
}