Manual:Externallinks table

The externallinks table is where MediaWiki stores and tracks external links. This table was introduced after 1.5.

el_id
The primary key. Presently not used for anything, but will help with online schema changes

el_from
The page identifier of the referring wiki page.

el_to
The actual URL itself. It is passed to the browser.

el_index
This is the same URL as  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'.

Current version
mysql> describe externallinks; +--+--+--+-+-++ +--+--+--+-+-++ +--+--+--+-+-++
 * Field   | Type             | Null | Key | Default | Extra          |
 * el_id   | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
 * el_from | int(10) unsigned | NO   | MUL | 0       |                |
 * el_to   | blob             | NO   | MUL | NULL    |                |
 * el_index | blob            | NO   | MUL | NULL    |                |

Previous versions
mysql> describe externallinks; +--+--+--+-+-+---+ +--+--+--+-+-+---+ +--+--+--+-+-+---+
 * 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 | NO   | MUL | 0       |       |
 * el_to   | blob            | NO   | MUL | NULL    |       |
 * el_index | blob           | NO   | MUL | NULL    |       |

+--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * 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 |         |       |