多个查询合并为一个查询答案

【问题标题】：Multiple queries into one query多个查询合并为一个查询
【发布时间】：2016-02-05 16:49:35
【问题描述】：

我正在尝试将多个查询合并为一个。有人有什么想法吗？

SELECT In_year, count(In_year) AS Num_Of_Year 来自实习 GROUP BY In_year;

SELECT In_qtr, count(In_qtr) AS Num_Of_Qtr 来自实习按季度分组；

SELECT In_location, count(In_location) AS Num_Of_Location 来自实习 GROUP BY In_location;

如果我自己运行每个查询，我会在结果中得到我想要的。我的问题是我无法弄清楚如何将这 3 个查询组合成 1 个，我试图将它们组合起来，但最终得到了错误的输出。谢谢！

【问题讨论】：

标签： mysql sql subquery

【解决方案1】：

也许 UNION 子句并稍微组织一下就可以解决问题：

SELECT 'Year' As Topic, In_year As Value, count(In_year) AS Count 
FROM INTERNSHIP 
GROUP BY In_year

UNION ALL

SELECT 'Qtr' As Topic, In_qtr As Value, count(In_qtr) AS Count 
FROM INTERNSHIP 
GROUP BY In_Qtr

UNION ALL

SELECT 'Location' As Topic, In_location As Value, count(In_location) AS Count
FROM INTERNSHIP 
GROUP BY In_location;

这是在单个查询中显示它的唯一方法，因为三个 SELECT 将具有不同的行数，因此您不能以自然形式添加列。

【讨论】：

谢谢！我最了解你的回答。

【解决方案2】：

我们可以使用 INTERNSHIP 表作为驱动列表来连接其他“group by”SQL：

select
   t1.In_year, y.Num_Of_Year,
   t1.In_qtr, q.Num_Of_Qtr,
   t1.In_location, l.Num_Of_Location
FROM
  (select distinct In_year, In_qtr, In_location from INTERNSHIP)  t1
   join (SELECT In_year, count(In_year) AS Num_Of_Year FROM INTERNSHIP GROUP BY In_year) y on t1.In_year = y.In_year
   join (SELECT In_qtr, count(In_qtr) AS Num_Of_Qtr FROM INTERNSHIP GROUP BY In_Qtr) q on t1.In_Qtr = q.In_Qtr
   join (SELECT In_location, count(In_location) AS Num_Of_Location FROM INTERNSHIP GROUP BY In_location) l on t1.In_location = l.In_location

【讨论】：