Dynamically change an SQL statement?

I'd like to have a standard sql select statement that depending on user input changes to alternative one.
I've written code similar to this:
if (something is true....)
{
SQLdataSRC1.SelectCommand = "Select *...  etc";
// where SQLdataSRC1 is the name of the sql select statement datasource in the front end code.
}
I've put this code in the onload event and in the SQL datasource event handler but still it gets ignored - and only the SQL select statement in the front end code is recognised.
What am I doing wrong?
...