I would recommend to follow below steps in order to resolve the connection refused error while connecting to SQL server through Denodo:
* Make sure there is no firewall blocking the requests to SQL server.
* Make sure the port used in the JDBC connection string is open.
* Make sure that the SQL server is in running state.
* Make sure the login credentials are correct and you are able to connect to the SQL server database with those login credentials.
For more information on this issue, you can refer to similar Q&As,
[IOException: Sql server: jdbc](https://community.denodo.com/answers/question/details?questionId=9060g0000000A0TAAU&title=IOException%3A+Sql+server%3A+jdbc) and [Unable to connect to SQL Server 2012](https://community.denodo.com/answers/question/details?questionId=90670000000k9m6AAA&title=Unable+to+connect+to+SQL+Server+2012)
I see that you have raised another [Q&A](https://community.denodo.com/answers/question/details?questionId=9060g0000004FOUAA2&title=connection+error+message) regarding the “Driver not found” error. So, if you are trying to connect to MS SQL server, you can use one of the SQL server adapters and provide the login details of the database. Since the driver for MS SQL is shipped with Denodo Express so this should be enough to establish a connection.
But in case you are trying to connect to MySQL database, you will have to do an additional step which is to download the JDBC driver for MySQL and paste it in `$Denodo_Home/lib-external/jdbc-drivers/MySql-5` folder.
Refer to the section Importing Relational Databases in this [document](https://community.denodo.com/tutorials/browse/basics/2fs5jdbc).
Hope this helps!