Что означают буквы перед названием столбцов таблиц базы данных?

$sql_select = "SELECT p.id, p.autor, p.date, p.short_story, CHAR_LENGTH(p.full_story) as full_story, p.xfields, p.title, p.category, p.alt_name, p.comm_num, p.allow_comm, p.fixed, p.tags, e.news_read, e.allow_rate, e.rating, e.vote_num, e.votes, e.view_edit, e.editdate, e.editor, e.reason FROM " . PREFIX . "_post p {$cat_join}LEFT JOIN " . PREFIX . "_post_extras e ON (p.id=e.news_id) WHERE {$stop_list}approve=1 AND allow_main=1" . $where_date . " ORDER BY " . $fixed . $news_sort_by . " " . $news_direction_by . " LIMIT " . $cstart . "," . $config['news_number'];