If you would like to test if the communication between DDC and SQL is working fine, outside Citrix, you can create an .udl file on desktop and try to access the SQL server using the required username and password. 

If the communication completes successfully, you can continue troubleshooting from Citrix perspective. 

If the communication fails, it means something else is blocking the communication between those servers (DDC and SQL) : SQL permissions, firewall, ports, etc. In this case you can refer customer to internal team or Microsoft in order to resolve it
 

Instructions

1. Create a file with extension “.udl “. For example: DBtest.udl
2. Open the DBtest.udl file.
3. Enter Server IP / FQDN.
4. Click on Refresh.
5. Enter the login information. Either use Windows NT Integrated Security or use a specific username and password.
6. Choose the database which you would like to check connectivity for.
7. Click on Test connection.
8. If the details entered are correct it displays the connection is succeeded.