+1 on Robin's answer to your question! But...you may have asked the wrong question
The ability to parameterize SQL statements is a great feature...but probably overkill based on your problem description.
It sounds like you're worried about your server operating on too many rows - understandable. Why not just add a filter to your worksheet or data source? It's much easier to deal with and does the same thing.
- Add a Filter on date to your worksheet....or
- Add a "Context filter" on date to your worksheet....or <--- Context Fillters = my favorite
- Add a Global filter/context filter to your workbook....or
- Add a filter directly to the data source (in version v8)
I generally suggest that people stay away from mucking with Custom SQL statements unless they are absolutely necessary and you are sure that the SQL you think will be more performant in Tableau actually is. A few more details here: http://tableaulove.tumblr.com/post/20781994395/tableau-performance-multiple-tables-or-custom-sql