Today I was thinking aloud about Tree Regular Expressions and how they might make a nice query language for document databases like CouchDB. Someone pointed out that CSS3 selectors might make a great concrete syntax for this. One thing lead to another and I thought, why not build a relational database in HTML? So I did. I even got inner joins working.

Let's start with a few tables:

<table class="users">
  <tr>
    <td class="id">1</td>
    <td class="first_name">amy</td>
    <td class="last_name">bobamy</td>
  </tr> 
  ...
</table>
<table class="photos">
  <tr>
    <td class="id">1</td>
    <td class="user_id">1</td>
    <td class="url">http://www.example.com/foo.png</td>
  </tr> 
</table>

Now we can express some queries:

$('.users')
  .where('.id:eq(1)')
  .select('*')

This is equivalent to SELECT * FROM users WHERE id = 1

$('.users')
  .where('.id:eq(1)')
  .select('.id, .name')

This is equivalent to SELECT id, name FROM users WHERE id = 1. Here is something slightly more complicated:

$('.users')
  .where('.name:contains(a)')
    .and('.name:contains(c)')
  .select('*')

But here is the crowning glory, the inner join:

$('.users')
  .join('.photos')
  .where('.photos.user_id:eq(.users.id)')
    .and('.users.id:eq(1)')
  .select('.photos.url')

This is equivalent to:

SELECT photos.url FROM users, photos
WHERE photos.user_id = users.id
  AND users.id = 1

Download the fun at Github.