How to use VIEW in WHERE clause (MySQL)? - mysql

Question

I want to use data of view in WHERE clause. But getting an error:
create view post_with_answers AS
SELECT DISTINCT postid
FROM (SELECT postid FROM `qa_posts` WHERE `type` = 'Q') AS q1
INNER JOIN (SELECT parentid FROM `qa_posts` WHERE `type` = 'A') AS q2 ON q1.postid = q2.parentid
select count(*)
from qa_posts
where parentid not in post_with_answers
On the last row I am getting this error:
SQL Error [1064] [42000]: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'post_with_answers' at line 3
How to fix that?

Answer 1

Just like you would use a table:
select count(*)
from qa_posts
where parentid not in (select pwa.postid from post_with_answers pwa);
I would caution you from using not in with a subquery. No rows are returned if even one value from the subquery is NULL. For this reason, I recommend NOT EXISTS:
select count(*)
from qa_posts p
where not exists (select 1
from post_with_answers pwa
where p.parentid = pwa.postid
);
In addition, your view definition is a bit over complicated. You don't need subqueries:
create view post_with_answers AS
SELECT DISTINCT pq.postid
FROM qa_posts pq JOIN
qa_posts pa
ON pq.postid = pa.parentid
WHERE pq.type = 'Q' AND pa.type = 'A';
Then, the DISTINCT just adds overhead, so EXISTS is better:
create view post_with_answers AS
SELECT DISTINCT pq.postid
FROM qa_posts pq
WHERE EXISTS (SELECT 1
FROM qa_posts pa
WHERE pq.postid = pa.parentid AND
pa.type = 'A'
)
WHERE pq.type = 'Q';