Question

这是我的疑问：

SELECT 
    log_timestamp, 
    system_name,
    faction_name,
    faction_allegiance, 
    faction_government_type, 
    current_state, 
    pending_state,
    recovering_state,
    influence AS influence,
    transactions.transaction_id
FROM `bgs`.`faction_log`
    INNER JOIN `factions` ON `factions`.`faction_id` = `faction_log`.`faction_id`
    INNER JOIN `transactions` ON `transactions`.`transaction_id` = `faction_log`.`transaction_id`
    INNER JOIN `system_log` ON `system_log`.`transaction_id` = `transactions`.`transaction_id`
    INNER JOIN `systems` ON `systems`.`system_id` = `transactions`.`associated_system_id` 
WHERE
    log_timestamp BETWEEN bgs.find_closest_tick_time(date('2017-08-14')) AND bgs.find_closest_tick_time(DATE_ADD(date('2017-08-14'), INTERVAL 1 DAY)) AND
    associated_system_id = 4 
GROUP BY 
    faction_name,
    log_timestamp,
    system_name,
    faction_allegiance, 
    faction_government_type, 
    current_state, 
    pending_state,
    recovering_state,
    influence,
    transactions.transaction_id

它工作正常，现在看起来像这样：

但我想只选择包含字段log_timestamp的最大值的行。（在这种情况下2017-08-15 06:09:52）。我试着用这样的东西：

HAVING transactions.log_timestamp >= max(date('2017-08-15 06:09:52'));

但它不会那样工作：

HAVING transactions.log_timestamp >= max(log_timestamp);

我做错了什么？

Answer 1

您可以尝试使用子查询，例如

WHERE
    log_timestamp = (
      SELECT MAX(log_timestamp) 
      FROM `bgs`.`faction_log`
      WHERE log_timestamp BETWEEN bgs.find_closest_tick_time(date('2017-08-14')) AND bgs.find_closest_tick_time(DATE_ADD(date('2017-08-14'), INTERVAL 1 DAY)) AND associated_system_id = 4
) AND associated_system_id = 4

Answer 2

这是另一个解决方案。如果您只想在选择列表的所有列中为每个不同的值设置一行，则不需要GROUP BY。您可以使用SELECT DISTINCT执行相同的操作，如下所示：

SELECT DISTINCT
    log_timestamp, 
    system_name,
    faction_name,
    faction_allegiance, 
    faction_government_type, 
    current_state, 
    pending_state,
    recovering_state,
    influence AS influence,
    transactions.transaction_id
FROM `bgs`.`faction_log`
    INNER JOIN `factions` ON `factions`.`faction_id` = `faction_log`.`faction_id`
    INNER JOIN `transactions` ON `transactions`.`transaction_id` = `faction_log`.`transaction_id`
    INNER JOIN `system_log` ON `system_log`.`transaction_id` = `transactions`.`transaction_id`
    INNER JOIN `systems` ON `systems`.`system_id` = `transactions`.`associated_system_id` 
WHERE
    log_timestamp BETWEEN bgs.find_closest_tick_time(date('2017-08-14')) AND bgs.find_closest_tick_time(DATE_ADD(date('2017-08-14'), INTERVAL 1 DAY)) AND
    associated_system_id = 4 
ORDER BY log_timestamp DESC
LIMIT 1

由于您只想要具有max log_timestamp的行，您可以按该列降序排序，并使用LIMIT仅返回结果中的第一行。

使MySQL查询选择group by语句的最大时间戳

2 个答案: