Question

I want to do a join accross a MS-SQL and MySql database.

The MS-SQL query generates an index, effectively, and then I want to pull all MySQL records back that match the result of this query. (I could bring back both tables, i.e. the unfiltered data from MySQL and then filter using Linq, but this will be inefficient as I’ll be pulling back loads more data than I need.)

The MS-SQL query is done via Linq:

var fb1 = from f in db.tl_feedbacks  
          where f.timestamp >= new DateTime(fromYear, fromMonth, fromDay)
             && f.timestamp <= new DateTime(toYear, toMonth, toDay)   
          select new {f.requestID, f.tl_feedback_score.score };

This will bring back a table like this:

RequestID | score
-----------------
12345     | 1
12349     | 3
12446     | 3

etc.

From this I want to return only those records from the following MySQL query that have a RequestID in the above table:

SELECT wo.WORKORDERID,
       COALESCE(ti.FIRST_NAME,'Not Assigned') AS 'Technician',
       COALESCE(cd.CATEGORYNAME, 'Not Assigned') AS Category,
       COALESCE(scd.NAME, 'Not Assigned') AS Subcategory,
       wof.UDF_CHAR1 "Office Location"  
FROM WorkOrder_Threaded wot  
INNER JOIN WorkOrder wo ON wot.WORKORDERID=wo.WORKORDERID  
LEFT JOIN SDUser sdu ON wo.REQUESTERID=sdu.USERID  
LEFT JOIN AaaUser aau ON sdu.USERID=aau.USER_ID  
LEFT JOIN WorkOrderStates wos ON wo.WORKORDERID=wos.WORKORDERID  
LEFT JOIN SDUser td ON wos.OWNERID=td.USERID  
LEFT JOIN AaaUser ti ON td.USERID=ti.USER_ID  
LEFT JOIN CategoryDefinition cd ON wos.CATEGORYID=cd.CATEGORYID  
LEFT JOIN SubCategoryDefinition scd ON wos.SUBCATEGORYID=scd.SUBCATEGORYID  
LEFT JOIN WorkOrder_Fields wof ON wo.WORKORDERID=wof.WORKORDERID  

i.e I only want to pull back records 12345, 12349 and 12446 in this example. Ultimately I want one single table which has the requestID, score, and the columns from the MySQL query. However, if I can get the “filtered” MySQL table back I can join the two afterwards. I just don’t want to bring the MySQL back “unfiltered” as the table will be huge.

Answer 1

With the right OLEDB database drivers (I’ve only done this with PGSQL so I can’t really advise), you can create a Linked Server in MSSQL. Here’s a walkthrough, and here’s another.

You can then query it using OPENQUERY as follows in MSSQL:

select * from openquery(LinkedServerDb,'select * from remotetable')

and join:

select 
    * 
from 
    openquery(LinkedServerDb,'select * from remotetable') remoteTable
    join localTable on remotetable.someid=localtable.otherid