I am having a three tables, for a sake of simplicity let's say
Category (1...many) Topic (1...many) Post
What I am trying to achieve is to get a CategoryID and a total number of topics in this category as well as the total number of posts.
The best result what I made was using the following query:
SELECT category.ID, COUNT(topic.id) AS topiccount, COUNT(post.id) AS postcount
FROM ((category)
LEFT JOIN topic ON topic.categoryid = category.id)
LEFT JOIN post ON post.topicid = topic.id
GROUP BY category.id
Unfortunately, even if I have just a 6 topics in the table associated with a category I am getting '7' as a result.
I did some research on it and it seems that I have to use a DISTINCT keyword inside a COUNT however access does not support it and I could not find appropriate way to do it in the subqueries.
Thank you for any help!
You get one more record, because you are not counting the topics, you are actually counting the topic-post joined records. Use the following :
SELECT category.id, count(topic.id), Nz(Sum(numofposts),0)
FROM (category LEFT JOIN (
SELECT topic.id, count(post.id) as numofposts, topic.categoryId
FROM topic LEFT JOIN post on topic.id = post.topicId
GROUP BY topic.id, topic.categoryId
) as TP ON category.id=TP.categoryid)
GROUP BY category.id
The Nz is there to ensure that in empty topics you don't get Null sums
Related
I was doing a practice question for SQL which asks to create a list of album titles and unit prices for the artist "Audioslave" and find out how many records are returned.
Here is the relational database picture given in the question:
Initially, I used an inner join to retrieve the list and actually got the correct answer (40 records returned). The code is shown below:
select a.Title, t.UnitPrice
from albums a
inner join tracks t on t.AlbumId = a.AlbumId
inner join artists ar on ar.ArtistId = a.ArtistId
where ar.Name = 'Audioslave';
Although I finished the question, I was curious to try to solve this problem using nested subqueries instead and tried to first retrieve the AlbumId and UnitPrice from tracks. I got the correct answer but not the correct list (the question asked for album title and not AlbumId). Here is the code:
select AlbumId, UnitPrice
from tracks
where AlbumId in (
select AlbumId
from albums
where ArtistId in (
select ArtistId
from artists
where Name = 'Audioslave'));
In order to solve the problem with the list, I tried combining the previous codes. However, I get a completely different amount of records being returned (10509).
select a.Title, t.UnitPrice
from albums a
inner join tracks t
where a.AlbumId in (
select AlbumId
from albums
where ArtistId in (
select ArtistId
from artists
where Name = 'Audioslave'));
I don't understand what I'm doing wrong with the last code...Any help would be appreciated! Also, sorry if I wrote too much, I just wanted to convey my thinking process clearly.
Some databases (SQLite, MySQL, Maria, maybe others) allow you to write an INNER JOIN without specifying ON, and they just cross every record on the left with every record on the right in that case. If there were 2 albums and 3 tracks, 6 rows would result. If the albums were A and B, and the tracks were 1, 2 and 3, the rows would be the combination of all: A1, A2, A3, B1, B2, B3
Other databases (Postgres, SQLServer, Oracle, maybe others) refuse to do it unless you specify ON. To get an "every row on the left combined with every row on the right" you have to write CROSS JOIN (or write an inner join with an ON that is always true)
It might help your mental model of what happens during a join to consider that the db takes all the rows on the left and connects them to all the rows on the right, then for each combination of rows, assesses the truth of the ON clause, and the WHERE clause, before deciding to return the row
For example, this will return 10509 rows:
SELECT * FROM albums INNER JOIN tracks ON 1=1
The on clause is always true
This will return 10509 tracks, but only if the query is run on Monday
SELECT * FROM albums INNER JOIN tracks ON strftime('%w', 'now') = 1
What goes in the ON or WHERE doesn't have to have anything to do with the data in the table.. it just has to be something that resolves to a Boolean
I have the following schema:
Multiple webinar entities can have multiple categories hence the webinarcategorymapping table.
What I need to achieve is find the most popular webinars (by likes number) of a specific category.
For doing this, I've written the query below:
select
webinar.id, webinar.name as "webinar", webinar.publishat,
string_agg(category.name, ',' order by category.name) as categories,
count("like".likeableid) as "likes_count"
from
webinar
join "like" on webinar.id = "like".likeableid and "like".likeabletype = 'webinar'
join webinarcategorymapping on webinarcategorymapping.webinarid = webinar.id
join category on category.id = webinarcategorymapping.categoryid
group by "like".likeableid, webinar.id
having
string_agg(category.name, ',' order by category.name) ilike '%CategoryName%'
and count("like".likeableid) > 0
order by count("like".likeableid) desc;
Due to the many-to-many relationship between category and webinar I've decided to join all categories for every webinar into a comma-separated value by using string_agg. This way I'll be able to perform the search by category by using ilike %search_term%.
In the like table the likeabletype must be equal to webinar and the likeableid filed is the id of an entity on which the like is made. So, in my case, when querying the like table I need to use likeabletype='webinar' and likeableid = webinar.id conditions.
The problem is that is gives me incorrect likes_count results (I guess it's due to multiple joins that duplicate many rows).
However using count(distinct "like".likeableid) doesn't help as it just gives me 1 for every row.
What should I change in my query in order to get correct result from count() of likes?
What I need to achieve is find the most popular webinars (by likes number) of a specific category.
You can aggregate the likes in a subquery and just filter on the categories:
select w.id, w.name as "webinar", w.publishat, num_likes
from webinar w join
(select l.likableid, count(*) as num_likes
from "like" l
where l.likeabletype = 'webinar'
group by l.likeableid
) l
on w.id = l.likeableid join
webinarcategorymapping wcm
on wcm.webinarid = w.id join
category c
on c.id = wcm.categoryid
where c.name = ?
order by num_likes desc;
I'm currently working on a database project and one of the problems calls for the following:
The Genre table contains twenty-five entries. The MediaType table contains 5
entries. Write a single SQL query to generate a table with three columns and 125
rows. One column should contain the list of MediaType names; one column
should contain the list of Genre names; the third column should contain a count of
the number of tracks that have each combination of media type and genre. For
example, one row will be: “Rock MPEG Audio File xxx” where xxx is the
number of MPEG Rock tracks, even if the value is 0.
Recognizing this, I believe I'll need to use a FULL OUTER JOIN, which Sqlite3 doesn't support. The part that is confusing me is generating the column with the combination. Below, I've attached the two methods I've tried.
create view T as
select MediaTypeId, M.Name as MName, GenreId, G.Name as GName
from MediaType M, Genre G
SELECT DISTINCT GName, MName, COUNT(*) FROM (
SELECT *
FROM T
OUTER LEFT JOIN MediaType
ON MName = GName
UNION ALL
SELECT *
FROM Genre
OUTER LEFT JOIN T
) GROUP BY GName, MName;
However, that returned nearly 250 rows and the GROUP BY or JOIN(s) is totally wrong.
I've also tried:
SELECT Genre.Name as GenreName, MediaTypeName, COUNT(*)
FROM Genre LEFT OUTER JOIN (
SELECT MediaType.Name as MediaTypeName, Track.Name as TrackName
FROM MediaType LEFT OUTER JOIN Track) GROUP BY GenreName, MediaTypeName;
Which returned 125 rows but they all had the same count of 3503 which leads me to believe the GROUP BY is wrong.
Also, here is a schema of the database:
https://www.dropbox.com/s/onnbwqfrfc82r1t/IMG_2429.png?dl=0
You don't use full outer join to solve this problem.
Because it looks like a homework problem, I'll describe the solution.
First, you want to generate all combinations of genres and media types. Hint: This uses a cross join.
Second, you want to count all the combinations that you have. Hint: this uses an aggregation.
Third, you want to combine these together. Hint: left join.
Here's what I want to achieve:
I have a number of categories, each one with products in it.
I want to produce a report that shows various information about those products for each category. So I have a query that looks something like:
select
category,
count(products),
sum(product_price),
from product
group by category
So far so good.
But now I also want to get some category-specific information from a table that has information by category. So effectively I want to say:
join category_info on category
except that that will create a join for each row of each group, rather than just one join for each group.
What I really want to be able to say to sql is 'for each group, take the distinct category value, of which there's guaranteed to only be one since I'm grouping on it, and then use that to join to the category info table'
How can I accomplish this in SQL? By the way, I'm using Oracle 10g..
Many thanks!
select a.category, a.Count, a.SumPrice
ci.OtherColumn
from (
select p.category,
count(p.products) as Count,
sum(p.product_price) as SumPrice,
from product p
group by category
) a
inner join category_info ci on a.category = ci.category
So I'm trying to build a simple forum. It'll be a list of topics in descending order by the date of either the topic (if no replies) or latest reply. Here's the DB structure:
Topics
id, subject, date, poster
Posts
id, topic_id, message, date, poster
The forum itself will consist of an HTML table with the following headers:
Topic | Last Post | Replies
What would the query or queries look like to produce such a structure? I was thinking it would involve a cross join, but not sure... Thanks in advance.
Of course you can make a query for this, but I advise you to create in Topics table fields 'replies' and 'last post', then update them on every new post. That could really improve your database speed, not now, but the time when you will have thousands of topics.
SELECT *
FROM
`Topics`,
(
SELECT *, COUNT(*) AS `replies`
FROM `Posts`
GROUP BY `Posts`.`topic_id`
ORDER BY `Posts`.`date` DESC
) AS `TopicPosts`
WHERE `Topics`.`id` = `TopicPosts`.`topic_id`
ORDER BY `Posts`.`date` DESC
This 'should' work, or almost work in the case it doesn't, but I agree with the other poster, it's probably better to store this data in the topics table for all sorts of reasons, even if it is duplication of data.
The forum itself will consist of an
HTML table with the following headers:
Topic | Last Post | Replies
If "Last Post" is meant to be a date, it's simple.
SELECT
t.id,
t.subject,
MAX(p.date) AS last_post,
COUNT(p.id) AS count_replies
FROM
Topics t
INNER JOIN Posts p ON p.topic_id = t.id
GROUP BY
t.id,
t.subject
If you want other things to display along with the last post date, like its id or the poster, it gets a little more complex.
SELECT
t.id,
t.subject,
aggregated.reply_count,
aggregated.distinct_posters,
last_post.id,
last_post.date,
last_post.poster
FROM
Topics t
INNER JOIN (
SELECT topic_id,
MAX(p.date) AS last_date,
COUNT(p.id) AS reply_count,
COUNT(DISTINCT poster) AS distinct_posters
FROM Posts
GROUP BY topic_id
) AS aggregated ON aggregated.topic_id = t.id
INNER JOIN Posts AS last_post ON p.date = aggregated.last_date
As an example, I've added the count of distinct posters for a topic to show you where this approach can be extended.
The query relies on the assumption that no two posts within one topic can ever have the same date. If you expect this to happen, the query must be changed to account for it.