That's not what SQL Expressions are used for.
When you connect Crystal directly to tables instead of to a command, you can use SQL expressions to do things like calling database functions using one or more fields. These then become part of the Select statement that Crystal generates to get the data. I've used them in a couple of different ways - for example, to truncate the time off of a datetime field or to call a custom function that returns a name based on an ID.
SQL Expressions can't replace commands, but they provide some of the power of commands to users who can't/don't want to develop their own SQL commands in Crystal.
-Dell