Stupid question...

How does an app like del.icio.us add tags to urls. They must have two tables in a database, tags and urls, but how do they associate tags with urls and visa versa?

 

Could be with a third table, a lookup table:

Table Tags:

ID - Tag Name
1 - Apple
2 - Digital Point

Table URLS:
ID - URL
1 - http://www.microsoft.com
2 - http://www.apple.com
3 - http://forums.digitalpoint.com
4 - http://www.digitalpoint.com/tools/

Table Lookup:
Tag ID - URL ID
1 - 2
2 - 3
2 - 4

Now if they want to display all the Digital Point tagged URLs:

SELECT url FROM table_urls LEFT JOIN table_lookup ON table_urls.id = table_lookup.url_id LEFT JOIN table_tags ON table_lookup.tag_id = table_tags.id WHERE table_tags.name = 'Digital Point';

Result:
http://forums.digitalpoint.com
http://www.digitalpoint.com/tools/

 

Thanks - exactly what I was looking for!