Deleting all revisions in WordPress without unintentional data loss. Source: http://www.ambrosite.com/blog/clean-up-wordpress-revisions-using-a-mysql-multi-table-delete

revisions.sql

DELETE a,b,c
FROM wp_posts a
LEFT JOIN wp_term_relationships b ON ( a.ID = b.object_id)
LEFT JOIN wp_postmeta c ON ( a.ID = c.post_id )
LEFT JOIN wp_term_taxonomy d ON ( b.term_taxonomy_id = d.term_taxonomy_id)
WHERE a.post_type = 'revision'
AND d.taxonomy != 'link_category';