The externallinks table is where MediaWiki stores and tracks external links. This table was introduced after 1.5.
+----------+-----------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-----------------+------+-----+---------+-------+
| el_from | int(8) unsigned | NO | MUL | 0 | |
| el_to | blob | NO | MUL | NULL | |
| el_index | blob | NO | MUL | NULL | |
+----------+-----------------+------+-----+---------+-------+
The el_from is the page identifier of the referring wiki page. el_to is the actual URL itself and is passed to the browser, whilst el_index is the same URL search-optimised: username and password information is stripped, and the other components are reversed for faster searching, so http://user:password@sub.example.com/page.html becomes http://com.example.sub./page.html , which allows searches of the form 'Show all links pointing to *.example.com'.
[edit] Previous versions
+----------+-----------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-----------------+------+-----+---------+-------+
| el_from | int(10) unsigned| NO | MUL | 0 | |
| el_to | blob | NO | MUL | NULL | |
| el_index | blob | NO | MUL | NULL | |
+----------+-----------------+------+-----+---------+-------+
+----------+-----------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-----------------+------+-----+---------+-------+
| el_from | int(8) unsigned | | MUL | 0 | |
| el_to | blob | | MUL | | |
| el_index | blob | | MUL | | |
+----------+-----------------+------+-----+---------+-------+
