In the interest of efficiency, you might want to provide a separate function for deleting all the rows from a table.

(defun delete-all-rows (table)

The remaining table operations don’t really map to normal relational database operations but will be useful in the MP3 browser application. The first is a function to sort the rows of a table in place.

On the flip side, in the MP3 browser application, you’ll need a function that shuffles a table’s rows in place using the function nshuffle-vector from Chapter 23.

  table)

With this code you’ll be ready, in Chapter 29, to build a Web interface for browsing a collection of MP3 files. But before you get to that, you need to implement the part of the server that streams MP3s using the Shoutcast protocol, which is the topic of the next chapter.

1The general theory behind interning objects is that if you’re going to compare a particular value many times, it’s worth it to pay the cost of interning it. The value-normalizer runs once when you insert a value into the table and, as you’ll see, once at the beginning of each query. Since a query can involve invoking the equality-predicate once per row in the table, the amortized cost of interning the values will quickly approach zero.

2As always, the first causality of concise exposition in programming books is proper error handling; in production code you’d probably want to define your own error type, such as the following, and signal it instead:

3If any MP3 files have malformed data in the track and year frames, **PARSE-INTEGER** could signal an error. One way to deal with that is to pass **PARSE-INTEGER** the argument of **T**, which will cause it to ignore any non-numeric junk following the number and to return **NIL** if no number can be found in the string. Or, if you want practice at using the condition system, you could define an error and signal it from these functions when the data is malformed and also establish a few restarts to allow these functions to recover.

4This query will also return all the songs performed by the Dixie Chicks. If you want to limit it to songs by artists other than the Dixie Chicks, you need a more complex :where function. Since the :where argument can be any function, it’s certainly possible; you could remove the Dixie Chicks’ own songs with this query:

This obviously isn’t quite as convenient. If you were going to write an application that needed to do lots of complex queries, you might want to consider coming up with a more expressive query language.