如何消除子查询以简化查询？

Question

I have three phone servers pushing the status of their extensions into MySQL on a regular basis. These systems are duplicates of each other in different geographic regions, but with the ability to fail over as needed. Therefore they will report the same extensions but only one of the three systems will show the extension is registered ("OK") at any one time.

For example, table is as follows:

+----------------+---------+----------+
| extension name | status  | systemid |
+----------------+---------+----------+
| 101            | OK      | PBX01    |
| 101            | UNKNOWN | PBX02    |
| 101            | UNKNOWN | PBX03    |
| 202            | UNKNOWN | PBX01    |
| 202            | OK      | PBX02    |
| 202            | UNKNOWN | PBX03    |
| 303            | UNKNOWN | PBX01    |
| 303            | UNKNOWN | PBX02    |
| 303            | OK      | PBX03    |
| 404            | UNKNOWN | PBX01    |
| 404            | UNKNOWN | PBX02    |
| 404            | UNKNOWN | PBX03    |
+----------------+---------+----------+

So extension 101 is registered to PBX01, 202 to PBX02, 303 to PBX03, and 404 is not registered to any PBX. No extension can be registered to more than one PBX at a time.

An OK status would be the "best" status and therefore the one I would wish to report, provided it exists. However if there is an extension such as 404 that is not registered to any PBX I need to report that fact too.

My desired output:

+----------------+---------+----------+
| extension name | status  | systemid |
+----------------+---------+----------+
| 101            | OK      | PBX01    |
| 202            | OK      | PBX02    |
| 303            | OK      | PBX03    |
| 404            | UNKNOWN |          |
+----------------+---------+----------+

I have come up with the following query which delivers the result I want:

SELECT * FROM 
    (SELECT * FROM table
     ORDER BY extension,status) AS dummytable
     GROUP BY extension

However this query is too complex for my web application. I wish to create a view with the results of this query, however I cannot create a view with a subquery in the FROM clause.

I tried creating two views: one containing the subquery:

SELECT * FROM table ORDER BY extension,status

and a second view containing:

SELECT * FROM view1 GROUP BY extension

However the results are invalid as they seem to ignore the ORDER BY in view1.

I also looked into a stored procedure however that doesn't work with my web application either.

Any ideas?

Answer 1

Have you tried adding the order by to the second view?

SELECT * FROM view1 GROUP BY extension ORDER BY extension

Here's another more complex option that I know will work in SQL Server, but am unsure about in mysql. This one fetches the OK extensions and then those extensions with no PBX (this is accomplished by selecting any extension that has a count of 3 to match the number of your extensions):

SELECT ExtensionName ,
       STATUS ,
       PBX
  FROM TABLE
 WHERE STATUS = 'OK'

 UNION

SELECT ExtensionName ,
       'UNKNOWN' ,
       ''
  FROM TABLE
 WHERE STATUS = 'UNKNOWN'
GROUP BY ExtensionName
HAVING COUNT(1) = 3

ORDER BY ExtensionName

If that syntax doesn't work in MySql or is too reliant on each extension being registered for each pbx and there being a total of 3 pbxs, the second select statement could be rewritten as:

SELECT ExtensionName ,
       'Unknown' ,
       ''
  FROM [TABLE]
 WHERE STATUS = 'UNKNOWN'
   AND NOT EXISTS(SELECT 1 
                    FROM TABLE t1
                   WHERE t1.ExtensionName = ExtensionName
                     AND STATUS = 'OK')

or:

SELECT ExtensionName ,
       'Unknown' ,
       ''
  FROM TABLE
 WHERE STATUS = 'UNKNOWN'
   AND ExtensionName NOT IN (SELECT ExtensionName
                               FROM TABLE
                              WHERE STATUS = 'OK')

Answer 2

您可以在一个查询中组合GROUP BY和ORDER BY。

SELECT * FROM table GROUP BY extension ORDER BY extension, status

Answer 3

I think if you only want the correct answers, this covers the states you've described:

select systemid, extension, status from test where status='OK' or (status='UNKNOWN' and systemid is null);

assuming that an unknown condition for an unknown extension creates null, if it creates an empty string, test accordingly.

Answer 4

You have to select column dynamically on condition on other column. I think this query will work.

Select 
    extension name, 
    status, 
    CASE 
        WHEN status ='OK' THEN systemid 
        ELSE NULL END
From table
ORDER BY status
GROUP BY extension

I havn't executed it. Please suggest if any problems are present with this query.

Answer 5

Join it to itself. To replicate the results of the query that you said worked and gave you your desired results:

SELECT DISTINCT t1.extension
    , IFNULL(t2.status, t1.status) AS status
    , IFNULL(t2.systemid, t1.systemid) AS systemid
FROM pbx AS t1 
    LEFT OUTER JOIN
    pbx AS t2
    ON t1.extension = t2.extension
        AND t2.status = 'OK'
GROUP BY t1.extension, t2.status
ORDER BY t1.extension

I did not create the query above based on the desired output you provided, because the systemid column in the output you posted didn't have a value for extension 404.