Can I compare the resultsets of two queries?

Yes. Starting with version 1.1.0 you can compare the data of two sql queries.

Can I compare data between different DBMSs running on different platfroms/computers? 

Yes. As long as you can connect to the databases you want to use via ODBC 3.5 compliant drivers. 

Where can I get ODBC 3.5 Drivers?

See Drivers

What features will be disabled after the evaluation period expires if I do not buy a license for the application?

After the 30-day evaluation period expires, some features of CompareData will be disabled (except for Firebird data sources). You can enable those features by buying a license for the application. A license is not required for comparing table/view data. A new evaluation period is started with each new release of the application.

Feature No License Evaluation Period/​Registered/​Firebird
Compare Table/View data Yes Yes
Query data   Yes
Metadata   Yes
Synchronize Table/View data   Yes
Save as SQL statements Yes
Save Updates as SQL Yes
Generate Data Sync Script Yes
Tables comparison scheam filter
I have added a new tables comparison but no table for my schema is shown in the results for the comparison

A new tables comparison uses your login ID as the default schema filter value for the comparison. To change a tables comparison schema filter, use the properties schema filter dropdown to select a specific schema or all schemas for the tables comparison. Depending on the DBMS, a tables comparison schema filter may be labeled as 'Schema' or 'Owner', etc.