empty matches 0 in sql????

page_id = '', the following sql still work.

SELECT SQL_CACHE a.unique_id, b.banner_id, a.page_id, a.grp, a.page_pos,
a.order_seq, b.banner_type, b.image_name, b.link, b.target, b.params, ''
AS sku_list
FROM page_banners_link a, page_banners b
WHERE a.page_id = ''
AND site_id = '0'
AND a.banner_id = b.banner_id
ORDER BY a.page_pos, a.page_id, a.order_seq
LIMIT 0 , 30