MySQL联合查询，按2个变量排序

Question

每个表（table1 & table2）都有自己的DATETIME字段。
我试图捕捉两个表的id，并按他们的DATETIME字段排序。

实施例：

Table 1      Table 2 
------------     ------------- 
id | datetime1    id | table1id | datetime2 
------------------------  ----------------------- 
1 | 2014-09-21 20:31:26  1 | 2   | 2014-09-21 20:31:29 
2 | 2014-09-21 20:31:27  2 | 3   | 2014-09-21 20:31:30 
3 | 2014-09-21 20:31:28 

Table 3      
------------    
id | user    
------------------------ 
2 | phil     
3 | nathalie 

我的输出不与此查询正确排序：

SELECT * 
FROM (
    SELECT 
    1 AS selection, 
    table1.id, table1.datetime1, 
    table2.datetime2 
    table3.user 
    FROM Table1 
    LEFT OUTER JOIN table2 
    ON table1.id = table2.table1id 
    LEFT OUTER JOIN table3 
    ON table1.id = table3.id 

    UNION ALL 

    SELECT 
    2 AS selection, 
    table1.id, table1.datetime1, 
    table2.datetime2 
    table3.user 
    FROM Table1 
    INNER JOIN table2 
    ON table1.id = table2.table1id 
    INNER JOIN table3 
    ON table1.id = table3.id 
) AS query 
ORDER BY table1.datetime1 DESC, table2.datetime2 DESC 

期望的数据：
从表2 ID：2，1，
从表1 id：3，2，1
So：2，1，3，2，1

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Answer 1

你真正需要做的是更仔细考虑您的架构。考虑命名的日期时间列相同，然后运行一个查询这样的 - http://sqlfiddle.com/#!2/a3b4c/7/0

SELECT selection, id, datetimefoo, user FROM (
    SELECT 
    1 AS selection, 
    table1.id, table1.datetimefoo, 
    table3.user 
    FROM table1 
    LEFT OUTER JOIN table2 
    ON table1.id = table2.table1id 
    LEFT OUTER JOIN table3 
    ON table1.id = table3.id 


    UNION 

    SELECT 
    2 AS selection, 
    table1.id, table1.datetimefoo, 
    table3.user 
    FROM table1 
    INNER JOIN table2 
    ON table1.id = table2.table1id 
    INNER JOIN table3 
    ON table1.id = table3.id 

) AS T2 
ORDER BY datetimefoo DESC 

在SQL拨弄这将产生的结果更接近你在找什么。我仍然不确定你为什么需要第二个查询中的INNER JOINS，但是这里没有你在做的事情。

这里是不需要的列名的变化的另一种方法，但需要排序的列一个别名 - http://sqlfiddle.com/#!2/ec4bc/3/0

SELECT * FROM (
    SELECT 
    1 AS selection, 
    table1.id, table1.datetimefoo AS sort_date, -- alias on first table's date 
    table2.datetimebar, 
    table3.user 
    FROM table1 
    LEFT OUTER JOIN table2 
    ON table1.id = table2.table1id 
    LEFT OUTER JOIN table3 
    ON table1.id = table3.id 


    UNION 

    SELECT 
    2 AS selection, 
    table1.id, table1.datetimefoo, 
    table2.datetimebar AS sort_date, -- alias on second table's date 
    table3.user 
    FROM table1 
    INNER JOIN table2 
    ON table1.id = table2.table1id 
    INNER JOIN table3 
    ON table1.id = table3.id 

) AS T2 
ORDER BY sort_date DESC 

Answer 2

我相信你是过于复杂一个相当简单的任务：

SELECT * 
FROM (
    SELECT 1 AS selection, table1.id as id, table1.datetime1 as date FROM Table1 
    UNION ALL 
    SELECT 2 AS selection, table2.id as id, table2.datetime2 as date FROM Table2 
) AS query 
ORDER BY date DESC