select c.*
from posts p
join comments c on (p.id=c.post_id)
and c.status = 'A'
and c.id >= (select MIN(id) from (select id from comments where post_id=p.id order by id DESC LIMIT 7) as c2)
where p.comments_count > 0 AND p.id IN (247,245,244,243,242,241)
In the above sql query I get this mysql error Unknown column 'p.id' in 'where clause' it seems like in the subquery p.id isn't able to get referenced. Is there anyway that I can get the posts table id referenced in this subquery?
I think you are trying to fetch latest 7 comments for each post. Could you try this? you can test here http://www.sqlfiddle.com/#!2/a222e/3/0
The First Attempt
I tried below SQL.
SELECT *
FROM comments t1
WHERE post_id IN (247,254,244,243,242,241)
AND id IN (
SELECT id
FROM comments
WHERE t1.id = id
LIMIT 7
);
But I got an error "This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery"
Another Approach
So, I tried self join on comments to generate sequence number.
SELECT id
FROM (
SELECT t1.id, COUNT(*) AS cnt
FROM comments t1 INNER JOIN comments t2
ON t1.post_id = t2.post_id
WHERE t1.id <= t2.id
AND t1.post_id IN (247,254,244,243,242,241)
AND t2.post_id IN (247,254,244,243,242,241)
GROUP BY t1.id
) x
WHERE cnt <= 7;
inner sub-query uses self join and produces cnt column which has sequential value for each comment id of post.
But preceding query only fetches id of comment
Finally to get all columns of comment table, following query should be executed.
SELECT *
FROM comments c INNER JOIN (
SELECT id
FROM (
SELECT t1.id, COUNT(*) AS cnt
FROM comments t1 INNER JOIN comments t2
ON t1.post_id = t2.post_id
WHERE t1.id <= t2.id
AND t1.post_id IN (247,254,244,243,242,241)
AND t2.post_id IN (247,254,244,243,242,241)
GROUP BY t1.id
) x
WHERE cnt <= 7
) t USING (id);
Using User Variables
Actually you have another chance using MySQL user variable. I didn't mention this interesting MySQL feature because I was not sure I understood your question correctly.
SELECT *
FROM (
SELECT post_id, id,
IF (#pid = post_id, #cnt := #cnt + 1, #cnt := 1) AS cnt,
#pid := post_id
FROM comments, (SELECT #pid := 0, #cnt := 0) tmp
WHERE post_id IN (247,254,244,243,242,241)
ORDER BY post_id, id DESC
) x
WHERE cnt <= 7;
Preceding SQL looks like simpler (means good performance) than older join version. but not tested on large data set.
You can only access the outer query 1 level deep. So try doing you where filter on the middle query:
SELECT MIN(id)
FROM (
SELECT id, post_id
FROM comments
ORDER BY id DESC
LIMIT 7
)
WHERE post_id = p.id
Related
I want to keep lastest N records of each user_id and delete others.
Structure table "tab":
id (auto increment)
user_id
information
If possible, I would like to not delete if a user's number of records is less than N.
Thank you in advance.
You can use correlated subquery as follows:
Delete from your_table t
Where N <= (select count(1)
from Your_table tt
where tt.id < t.id)
You can use join in a delete:
delete t
from t join
(select t.*, row_number() over (order by id desc) as seqnum
from t
) tt
on tt.user_id = t.user_id
where seqnum > N;
This reversely enumerates the rows for a given user_id and then deletes those whose enumeration is too large.
I should add that this requires MySQL 8+.
EDIT:
In older versions of MySQL, you can use:
delete t
from t join
(select u.user_id,
(select t2.id
from t t2
where t2.user_id = t.user_id
order by t2.id desc
limit 1 offset N
) as nth_id
from (select distinct user_id from t) u
) tt
on tt.user_id = t.user_id
where t.id <= nth_id;
Test the subquery before you run the delete. It should be returning the n+1th id for each user.
I got two table users(table 01)、record_dcm_upload(table02)
i try to query counts and latest upload file time by everylogin account(users.username)
like
SELECT record_dcm_upload.user_id, users.username, record_dcm_upload.upload_time, COUNT( * )
FROM record_dcm_upload
JOIN users ON ( users.id = record_dcm_upload.user_id )
GROUP BY record_dcm_upload.user_id
but my query sql got some problem (actually the result of upload_time not the latest)
how should i adjust my query code (hope user_id and upload_time all sort By DESC)
SELECT t2.id, t2.username, MAX(t1.upload_time), COUNT(*)
FROM record_dcm_upload t1
JOIN users t2 ON ( t2.id = t1.user_id )
GROUP BY t2.id, t2.username
I have the following query:
SELECT c.text1, sum(c.num1), sum(c.num2), sum(c.num3),
(SELECT count(id) FROM table2 WHERE type = 1 AND txt = c.text1 AND spec_id = c.sp_id)
FROM table1 as c
WHERE c.type = 1
GROUP BY c.text1, c.sp_id
Is there a workaround to loose the c.sp_id from the GroupBy clause somehow? I know that if I remove it MySQL will return an error.
Or is there a way to group the results of this query by c.text1 only?
If I understand the problem correctly, you need to do two separate aggregations. This is one version of the query:
SELECT c.text1, c.sum1, c.sum2, c.sum3, t2.cnt
FROM (SELECT c.text1, sum(c.num1) as sum1, sum(c.num2) as cum2, sum(c.num3) as sum3
FROM table1 c
GROUP BY c.text1
) c LEFT JOIN
(SELECT txt, count(*) as cnt
FROM table2 t2
WHERE t2.type = 1 AND
EXISTS (SELECT 1
FROM table1 c2
WHERE t.txt = c2.txt AND c2.type = 1 AND
t.spec_id = c2.sp_id
)
) t2
ON t2.txt = c.text1
WHERE c.type = 1;
I'm trying to do an subquery with SUM() and LIMIT. This works fine with the following code:
SELECT id,
(
SELECT SUM(number)
FROM (
SELECT number
FROM t2
WHERE u_id = '1'
ORDER BY time ASC
LIMIT 30
) AS temp
) AS test
FROM t1
But I want to do it of course dynamically and with the current row ID.
I changed the Query to the following:
SELECT id,
(
SELECT SUM(number)
FROM (
SELECT number
FROM t2
WHERE u_id = p.id
ORDER BY time ASC
LIMIT 30
) AS temp
) AS test
FROM t1 p
This will give the following error:
Unknown column 'p.id' in 'where clause'
Any ideas how to make it working?
Unfortunately, MySQL limits the scope of table aliases. Oracle is another database that does this.
You can phrase your query as a complicated join:
select t1.id, sum(t2.number)
from t1 p join
t2
on p.id = t2.u_id
where 30 >= (select count(*)
from t2 t22
where t22.u_id = t2.u_id and
t22.time <= t2.time
)
group by t1.id;
Or you can do this with variables:
select p.id, sum(number)
from t1 p join
(select t2.*,
#rn := if(#u = t2.u_id, #rn + 1, if((#u := t2.u_id) is not null, 1, 0)) as rn
from t2
(select #u := 0, #rn := 0) vars
order by t2.u_d, time
) t2
on p.id = t2.u_id
where rn <= 30
group by p.id;
why not just change p.id to t1.id? I'm pretty sure it's because you are aliasing t1 in the first select, and it isn't defined in the subquery. Try an inner join instead.
SELECT id,
(
SELECT SUM(number)
FROM (
SELECT number
FROM t2
INNER JOIN t1 p
on u_id = p.id
ORDER BY time ASC
LIMIT 30
) AS temp
) AS test
FROM t1 p
Try this:
SELECT id, temp2.sum_number as test
FROM t1 p
INNER JOIN
(
SELECT SUM(number) as sum_number, temp.u_id
FROM (
SELECT number, u_id
FROM t2
WHERE u_id = p.id
ORDER BY time ASC
LIMIT 30
) AS temp
) AS temp2 ON temp2.u_id = p.id
I moved subqueries in the join part, so i can access to p.id in the subquery.
I wrote this:
SELECT DISTINCT CATEGORY FROM T AS T1
CROSS JOIN (SELECT *
FROM T
WHERE T.CATEGORY = T1.CATEGORY
ORDER BY CATEGORY DESC
LIMIT 10)
and I receive this
"Unknown column 'T1.CATEGORY' in 'where clause'".
Why?
Update:
My purpose of this is to get 10 posts of any category.
Because T1 is not visible from within the subquery.
Your JOIN also serves no purpose and/or you probably forgot the JOIN condition.
In JOIN condition should use ON keyword
SELECT DISTINCT CATEGORY FROM T AS T1
CROSS JOIN SELECT * FROM T ON T.CATEGORY = T1.CATEGORY
ORDER BY CATEGORY DESC LIMIT 10;
If you need to get 10 posts of each category you can use a query like this:
SELECT CATEGORY, Post
FROM (
SELECT a.CATEGORY, a.Post, count(*) as rn
FROM #T a
JOIN #T b ON a.CATEGORY = b.CATEGORY AND a.Post >= b.Post
GROUP BY a.CATEGORY, a.Post) dt
WHERE rn < 11;