I asked this question earlier, and found that there is no exact match search in the GUI. However, it can be done using a compound query. In your example the search string
Code:
tag:"science" and not tag:"science fiction"
will give you what you want.
You can use the tag editor available from the edit metadata dialog to delete tags using exact match. This helps if one accidentally creates a tag that matches almost everything, such as 'e' (I once did this.
).
Edit: Ooops. Just noticed that I repeated itimpi's advise. Apologies for the extra traffic.