mysql> explain select guid from workers_st where sguid=419946 and guid not in (select item from post_st where guid=419946);
+----+--------------------+---------------------+-----------------+---------------+------------+---------+-------+------+--------------------------+
| id | select_type        | table               | type            | possible_keys | key        | key_len | ref   | rows | Extra                    |
+----+--------------------+---------------------+-----------------+---------------+------------+---------+-------+------+--------------------------+
|  1 | PRIMARY            | workers_st          | ref             | sguid         | sguid      | 4       | const |  215 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | post_st             | unique_subquery | PRIMARY,guid  | PRIMARY    | 4       | func  |    1 | Using where              |
+----+--------------------+---------------------+-----------------+---------------+------------+---------+-------+------+--------------------------+

select b.guid from post_st a join workers_st b on b.sguid=419946 and a.guid=419946 and b.guid not in (a.item);

SELECT
        mail_st.name
	workers_st.guid
FROM
	workers_st
LEFT JOIN post_st ON workers_st.guid = post_st.item AND post_st.guid = 419946
INNER JOIN mail_st ON mail_st.entry = post_st.mailentry
WHERE
	workers_st.sguid = 419946
AND post_st.item IS NULL

workers_st (guid, sguid)
post_st (guid, item, mailentry)
mail_st (entry,name)