from http://www.valipetcu.com/2010/re-attaching-images-to-posts-in-wordpress.html

Script-for-reattaching-WordPress-attachments-to-posts.sql

delimiter //
CREATE PROCEDURE reattach_images()
BEGIN
    DECLARE _attach_id BIGINT DEFAULT 0;
    DECLARE _attach_guid VARCHAR(255);
    DECLARE _parent_id BIGINT DEFAULT 0;
    DECLARE _done INT DEFAULT 0;

    BEGIN
        DECLARE cur_attach CURSOR FOR SELECT ID, guid FROM wp_posts WHERE post_parent = 0 AND post_type = 'attachment';
        DECLARE CONTINUE HANDLER FOR NOT FOUND SET _done = 1;
        OPEN cur_attach;

        REPEAT
            FETCH cur_attach INTO _attach_id, _attach_guid;
   
            IF (_done = 0) THEN
                SELECT ID INTO _parent_id FROM wp_posts WHERE post_content LIKE CONCAT('%', _attach_guid, '%') AND post_type = 'post' LIMIT 0,1;

                     IF (_parent_id IS NOT NULL) THEN
                    UPDATE wp_posts SET post_parent = _parent_id WHERE ID = _attach_id AND post_type = 'attachment';                                END IF;
                SET _done = 0;

            END IF;
            UNTIL (_done = 1)
        END REPEAT;
        CLOSE cur_attach;
        SET _done = 0;
    END;
END //

CALL reattach_images();