MySQL UNION - 每个派生表都必须有自己的别名答案

【问题标题】：MySQL UNION - Every derived table must have its own aliasMySQL UNION - 每个派生表都必须有自己的别名
【发布时间】：2014-10-10 01:10:01
【问题描述】：

我正在寻找解决方案：

SELECT SUM(`quant`), MONTH(`date`) AS month, `id` from (
(SELECT `date`, `id`, count(`hit`) AS `quant ` FROM `stat_2014_07` WHERE `k_id` = '123') 
UNION ALL 
(SELECT `date`, `id`, count(`hit`) AS `quant ` FROM `stat_2014_08` WHERE `k_id ` = '123') 
) group by id, month

MySQL：每个派生表都必须有自己的别名

【问题讨论】：

您可以添加一个别名，方法是在关闭 ) 后为子查询指定一个名称。所以，像(SELECT foo FROM bar) thisisaderivedtablealias 这样的东西。您可以在 UNION ALL BTW 中省略 2 个内部查询周围的 ()。
具有相同结构但名称不同的表通常表示数据库设计不佳。如果您有一个名为stats 的表包含所有月份的数据，则不需要union all。

标签： mysql sql select group-by union

【解决方案1】：

正是错误消息所说的内容。在您的（简化的）查询中：

SELECT SUM(`quant`), MONTH(`date`) AS month, `id` 
from (
  ... inner select 
) 
group by id, month;

您没有为派生表指定别名。所以应该是：

SELECT SUM(`quant`), MONTH(`date`) AS month, `id` 
from (
  ... inner select 
) as t -- this is the change
group by id, month;

顺便说一句：联合的选定部分周围的括号完全没用。为了清楚起见，我建议删除它们：

SELECT SUM(`quant`), MONTH(`date`) AS month, `id` 
from (
  SELECT `date`, `id`, count(`hit`) AS `quant ` FROM `stat_2014_07` WHERE `k_id` = '123'
  UNION ALL 
  SELECT `date`, `id`, count(`hit`) AS `quant ` FROM `stat_2014_08` WHERE `k_id ` = '123'
) as t -- this is the change
group by id, month;

【讨论】：

【解决方案2】：

您需要为查询指定别名：

SELECT   SUM(`quant`), MONTH(`date`) AS month, `id` 
FROM     ((SELECT `date`, `id`, count(`hit`) AS `quant` 
           FROM   `stat_2014_07` 
           WHERE  `k_id` = '123') t1
          UNION ALL 
          (SELECT `date`, `id`, count(`hit`) AS `quant` 
           FROM   `stat_2014_08` 
           WHERE  `k_id ` = '123') t2
         ) t_union
GROUP BY id, month

【讨论】：