SQL Aplication Issue

This is not a technical problem, but an application sql problem for which we are not too aware of the solution...

Currently we have two identical tables sol_a and sol_b and a master table that contains the version currently used, i.e. a or b for queries.

At any point in time a or b is empty and a query runs that gets data from both by doing a union....

We would like to have a view or a query that does a select from onlythe relevant table, selecting the table by looking at the parameter in the master table, ie 'a' or 'b'.  We have not come up with a solution, and would be grateful for assistance. Also, please guide as to what to do in the future when we have non techical queries like this that are non severe...