The only way to create a current selection with the SQL engine is to use the QUERY BY SQL command. There is no difference between a SQL current selection of records and a 4D current selection. In other words, creating a new current selection in one language wipes out what the current selection from the other language created.
For example, to show a selection of planets whose radius is greater than 6,000 km:
|QUERY BY SQL([Planets];"radius < 6000") |
Once this current selection is made, you can operate on it as you normally would using the 4D language.
See the QUERY BY SQL documentation for more information.