MySQL联合查询，按2个变量排序

Question

Each table (table1 & table2) has its own DATETIME field.
I'm trying to catch id's of the two tables and order them by their DATETIME field.

Example:

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

My output isn't ordered properly with this query:

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 

Desired data:
from table 2 id: 2, 1,
from table 1 id: 3, 2, 1
So: 2, 1, 3, 2, 1

////EDIT

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////EDIT

Answer 1

What you really need to do is to consider your schema more carefully. Consider naming the date time columns the same and then running a query like this - 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

In the SQL fiddle this produces the results closer to what you're looking for. I am still not sure why you need the INNER JOINS on the second query though - there is nothing that you're doing here whcih requires them.

Here is another method that does not require a changing of the column names, but requires an alias for the sortable columns - 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