I am having some trouble with a query that involves sum, a left join and a group by
$sql = "
SELECT h.id AS hid
, SUM(l.likes) AS likes
FROM homes h
LEFT
JOIN likes l
ON l.homeid = h.id
GROUP
BY h.id
";
Instead of summing the likes for each home, it is giving NULL if the home has no likes or the number 8873 if it has one like. I really can't understand the 8873.
Of note, there are a lot of likes in this table for other things in which case the value for l.homeid is NULL. Could that be throwing things off?
Edit:
I added another like for a homeid from a different user and now it is giving me 8906 instead of 8873 for those with 1 like and 17812 for the one with two likes. This is very strange. The data type for all the numbers is int(11). I am going to create a totally new table and see if that one does the same thing. I'm also going to remove a unique index I added recently.
Try moving the aggregation into a derived table and joining then joining that:
SELECT
h.id AS hid
, COALESCE( l.likes, 0 ) AS likes
FROM homes h
LEFT JOIN (
SELECT
homeid
, COUNT( likes ) AS likes
FROM likes
GROUP BY
homeid
) AS l ON h.id = l.homeid
Also try COUNT() instead of SUM()
Related
I have this query I need to optimize further since it requires too much cpu time and I can't seem to find any other way to write it more efficiently. Is there another way to write this without altering the tables?
SELECT category, b.fruit_name, u.name
, r.count_vote, r.text_c
FROM Fruits b, Customers u
, Categories c
, (SELECT * FROM
(SELECT *
FROM Reviews
ORDER BY fruit_id, count_vote DESC, r_id
) a
GROUP BY fruit_id
) r
WHERE b.fruit_id = r.fruit_id
AND u.customer_id = r.customer_id
AND category = "Fruits";
This is your query re-written with explicit joins:
SELECT
category, b.fruit_name, u.name, r.count_vote, r.text_c
FROM Fruits b
JOIN
(
SELECT * FROM
(
SELECT *
FROM Reviews
ORDER BY fruit_id, count_vote DESC, r_id
) a
GROUP BY fruit_id
) r on r.fruit_id = b.fruit_id
JOIN Customers u ON u.customer_id = r.customer_id
CROSS JOIN Categories c
WHERE c.category = 'Fruits';
(I am guessing here that the category column belongs to the categories table.)
There are some parts that look suspicious:
Why do you cross join the Categories table, when you don't even display a column of the table?
What is ORDER BY fruit_id, count_vote DESC, r_id supposed to do? Sub query results are considered unordered sets, so an ORDER BY is superfluous and can be ignored by the DBMS. What do you want to achieve here?
SELECT * FROM [ revues ] GROUP BY fruit_id is invalid. If you group by fruit_id, what count_vote and what r.text_c do you expect to get for the ID? You don't tell the DBMS (which would be something like MAX(count_vote) and MIN(r.text_c)for instance. MySQL should through an error, but silently replacescount_vote, r.text_cbyANY_VALUE(count_vote), ANY_VALUE(r.text_c)` instead. This means you get arbitrarily picked values for a fruit.
The answer hence to your question is: Don't try to speed it up, but fix it instead. (Maybe you want to place a new request showing the query and explaining what it is supposed to do, so people can help you with that.)
Your Categories table seems not joined/related to the others this produce a catesia product between all the rows
If you want distinct resut don't use group by but distint so you can avoid an unnecessary subquery
and you dont' need an order by on a subquery
SELECT category
, b.fruit_name
, u.name
, r.count_vote
, r.text_c
FROM Fruits b
INNER JOIN Customers u ON u.customer_id = r.customer_id
INNER JOIN Categories c ON ?????? /Your Categories table seems not joined/related to the others /
INNER JOIN (
SELECT distinct fruit_id, count_vote, text_c, customer_id
FROM Reviews
) r ON b.fruit_id = r.fruit_id
WHERE category = "Fruits";
for better reading you should use explicit join syntax and avoid old join syntax based on comma separated tables name and where condition
The next time you want help optimizing a query, please include the table/index structure, an indication of the cardinality of the indexes and the EXPLAIN plan for the query.
There appears to be absolutely no reason for a single sub-query here, let alone 2. Using sub-queries mostly prevents the DBMS optimizer from doing its job. So your biggest win will come from eliminating these sub-queries.
The CROSS JOIN creates a deliberate cartesian join - its also unclear if any attributes from this table are actually required for the result, if it is there to produce multiples of the same row in the output, or just an error.
The attribute category in the last line of your query is not attributed to any of the tables (but I suspect it comes from the categories table).
Further, your code uses a GROUP BY clause with no aggregation function. This will produce non-deterministic results and is a bug. Assuming that you are not exploiting a side-effect of that, the query can be re-written as:
SELECT
category, b.fruit_name, u.name, r.count_vote, r.text_c
FROM Fruits b
JOIN Reviews r
ON r.fruit_id = b.fruit_id
JOIN Customers u ON u.customer_id = r.customer_id
ORDER BY r.fruit_id, count_vote DESC, r_id;
Since there are no predicates other than joins in your query, there is no scope for further optimization beyond ensuring there are indexes on the join predicates.
As all too frequently, the biggest benefit may come from simply asking the question of why you need to retrieve every single row in the tables in a single query.
I am making a query but it is not working properly.
My table details are as follows:
Subarea:
id
Fieldsofstudy
student_subarea:
id primary key,
student_id ,
student_subarea Foreign key to subarea id and the student_subarea.
ASK:
What I want to accomplish is to obtain all fields of study in one column and in another column the id of the student if he is in the class. Otherwise, show null or something.
SELECT a.`id` , a.`name` , a.`area_id` , u. *
FROM `subarea` a
LEFT JOIN student_subarea u ON u.subarea_id = a.id
WHERE u.student_id =50
OR u.student_id IS NULL
Doing this is not helping at all. I tried to use functions and subqueries without any success. Could some help me.
The general rule for left join and filtering is to put the filtering clauses in the on clause for all but the first table. So this may do what you want:
SELECT a.`id`, a.`name`, a.`area_id`, u. *
FROM `subarea` a LEFT JOIN
student_subarea u
ON u.subarea_id = a.id AND u.student_id = 50;
How do you remember this logic? A left join returns all rows from the first table even when there is no match on the second table. That appears to be what you want.
The problem with your logic is that students other than student 50 match the logic. So, those rows get filtered out.
Move the filter criteria to the JOIN
SELECT a.`id` , a.`name` , a.`area_id` , u. *
FROM `subarea` a
LEFT JOIN student_subarea u
ON u.subarea_id = a.id
AND u.student_id =50
You should put the condition on the student ID inside the join condition, not in the where clause:
SELECT a.id, a.name , a.area_id, u.*
FROM subarea a
LEFT JOIN student_subarea u
ON u.subarea_id = a.id
AND u.student_id = 50
I'm finding trouble finding a similar example to what I'm trying to achieve. I have 3 tables. From one table I want to get the linking ID number. From another table I want to find the same ID's and add up another column of numbers in that table where the ID number from the 1st table matches. Then on the 3rd table, which is text, I want to group all the text together where the ID matches the main ID number... and return all this in 1 go. My diagram should show what I mean:
So have 2 queries that will on their own return part the results, but Im struggling to build it into 1 single query.
SELECT ticket_charges.ticket_id
, sum(ticket_charges.charge_time) AS Seconds
FROM
ticket_charges
LEFT OUTER JOIN tickets
ON ticket_charges.ticket_id = tickets.id
GROUP BY
ticket_charges.ticket_id
, tickets.id
The 77 and 937 for ticket ID 3 have been added up correctly!!
SELECT tickets.id AS `Ticket Number`
, left(tickets_messages.message, 500) AS `Ticket Message`
FROM
tickets
INNER JOIN tickets_messages
ON tickets.id = tickets_messages.id
GROUP BY
tickets_messages.ticket_id
, tickets.id
The messages are joined together correctly.
I've tried some concatenation on messages, selects within selects, different methods to group by, a couple of sums etc.. but just can't seem to get a result where by the I'm getting the results back correctly with both queries as 1 single query. Either the joined numbers from "charge_time" are very wrong and don't match any resemblance to anything or I end up with hundreds of "message" and strange numbers on the "charge_time"
FYI.. If I try this, I get "Sub query returned more than 1 row" but it's what I thought I should be doing.
SELECT ticket_charges.ticket_id
, sum(ticket_charges.charge_time) AS Seconds
FROM
ticket_charges
LEFT OUTER JOIN tickets
ON ticket_charges.ticket_id = tickets.id
Where (SELECT left(tickets_messages.message, 500)
FROM
tickets
INNER JOIN tickets_messages
ON tickets.id = tickets_messages.id
GROUP BY
tickets.id)
GROUP BY
ticket_charges.ticket_id
, tickets.id
If you really need to do that with a single query, the solution is to do a subquery in one of the jointures.
SELECT t.id, t.person_id, SUM(tc.charge_time), mc.concat
FROM tickets t
INNER JOIN tickets_charges tc ON tc.ticket_id = t.id
INNER JOIN (
SELECT ticket_id, GROUP_CONCAT(message SEPARATOR ' ') as concat
FROM tickets_messages
GROUP BY ticket_id) AS mc
ON mc.ticket_id = t.id
GROUP BY t.id
Try this query -
SELECT
t.id,
t.person_id,
SUM(tc.charge_time) Seconds,
GROUP_CONCAT(LEFT(tm.message, 20)) Message
FROM
tickets t
LEFT JOIN ticket_charges ts
ON ts.ticket_id = t.id
LEFT JOIN tickets_messages tm
ON tm.ticket_id = t.id
GROUP BY
t.id;
Note, that I used 'LEFT(tm.message, 20)', because GROUP_CONCAT function has length limitation - group_concat_max_len.
I have three tables that I need get information from, 1 table has the information in and the other two hold information that i need to count.
so the first tables structure is:
tbl_img
img_id
img_name
tbl_comments
comment_id
img_id
comment
tbl_vote
vote_id
logo_id
I want the results to have the count of comments and votes that relate to each logo.
I have a bit of the query which is for the count of comments, but have no idea for the syntax for the second join.
SELECT l.img_id, l.img_name, COUNT(c.comment_id) AS comment_count
FROM tbl_images as l
LEFT OUTER JOIN tbl_comments AS c USING (img_id);
Can anyone help?
how about this :
SELECT l.img_id, l.img_name,
(SELECT COUNT(*) FROM tbl_comments c WHERE i.img_id = c.img_id ) AS comment_count,
(SELECT COUNT(*) FROM tbl_vote v WHERE i.img_id = v.img_id ) AS vote_count
FROM tbl_images i
Sounds like you need two queries for this: One for counting the votes, and one for counting the comments.
As far as I know, COUNT counts result rows, and joins create result rows to display all allowed permutations of joined tables.
Assuming you have I entries, each with J comments and K votes, you would receive J*K rows for each entry after joins, and COUNTs would both return that J*K instead of the correct amount.
I do not remember if you can do inner queries in MySQL, but that would be the way to go.
(See #Kevin Burtons answer)
I have a db structure like:
posts
id
title
content
users
id
....
post_reads
post_id
user_id
How can I count the number of posts for which a particular user with an id say, x does not have a read record.
My SQL query currently looks like:
SELECT COUNT(posts.id) AS c
FROM `posts`
LEFT JOIN `post_reads` ON (`posts`.`id` = `post_reads`.`post_id`)
LEFT JOIN `users` ON (post_reads.user_id = `users`.`id` AND post_reads.user_id = x)
WHERE users.id IS NULL
AND post_reads.user_id IS NULL
I know I'm doing something wrong, although I'm not sure what that is.
This should to the trick
SELECT COUNT(posts.id) AS c
FROM posts
LEFT JOIN post_reads ON posts.id = post_reads.post_id AND post_reads.user_id = x
LEFT JOIN users ON post_reads.user_id = users.id
WHERE users.id IS NULL
Note that if you're not interested in doing anything with table users you can shorten this query to:
SELECT COUNT(posts.id) AS c
FROM posts
LEFT JOIN post_reads ON posts.id = post_reads.post_id AND post_reads.user_id = x
WHERE post_reads.user_id IS NULL
The first join you were doing is really an inner join, because it will never 'misfire'.
The second join will sometimes misfire, because you have the extra condition in there.
Therefore using the post_reads.some_id is null will never be true.
In order for that to work you'd have to repeat the AND post_reads.user_id = x in that join condition as well, but putting it in twice is silly and not needed, once will do.
PS don't forget to replace the 'x' with something more useful :-)
I tried this a few ways just using JOINS/WHERE, but they tend to miss certain cases (i.e. you can exclude posts joined to a read record for the given user, but the posts' ids will still be returned if they also join to read records for other users).
The simplest way may be something like this:
SELECT COUNT(DISTINCT id)
FROM posts
WHERE id NOT IN (SELECT DISTINCT post_id FROM post_reads WHERE user_id = #x)
Also, note that I don't believe you need to surround identifiers in backticks unless they are MySQL keywords.