Intersection in mysql

my first query

SELECT ID FROM wp_posts 
WHERE post_type = 'post' AND post_status = 'publish' 
AND ID IN (Select object_id FROM wp_term_relationships, wp_terms
           WHERE wp_term_relationships.term_taxonomy_id =18开发者_运维技巧) 
ORDER BY post_date DESC

produce result

 31 and 28

and my second query

SELECT ID FROM wp_posts 
WHERE post_type = 'post' AND post_status = 'publish' 
AND ID IN (Select object_id FROM wp_term_relationships, wp_terms 
           WHERE wp_term_relationships.term_taxonomy_id =8) 
ORDER BY post_date DESC

will produce

31 and 33

I need to get intersection of both query result.ie, 31 only. how can we get it using mysql

An intersect is nothing more than an inner join, actually:

select
    a.ID as ID
from
    (SELECT ID FROM wp_posts WHERE post_type = 'post' AND post_status = 'publish' AND ID IN (Select object_id FROM wp_term_relationships, wp_terms WHERE wp_term_relationships.term_taxonomy_id =8 or wp_term_relationships.term_taxonomy_id =18)) as a,
    (SELECT ID FROM wp_posts WHERE post_type = 'post' AND post_status = 'publish' AND ID IN (Select object_id FROM wp_term_relationships, wp_terms WHERE wp_term_relationships.term_taxonomy_id =18 or wp_term_relationships.term_taxonomy_id =18)) as b
where
    a.ID=b.ID

From the top of my head. I just copy & pasted your queries, btw.

One simple way would be like this:

SELECT FROM (that entire first query) WHERE ID IN (that entire second query)

I make no promises about its efficiency...

Your query is somehow not clear. Look at the second query. You have the following

wp_term_relationships.term_taxonomy_id =18   OR
wp_term_relationships.term_taxonomy_id =18

hmmm intersection - try inner join :P