Let's assume I have a database with two tables: categories and articles. Every article belongs to a category.
Now, let's assume I want to fetch the latest article of each category that fits a specific criteria (read: the article does). If it weren't for that extra criteria, I could just add a column called last_article_id or something similar to the categories table - even though that wouldn't be properly normalized.
How can I do this though? I assume there's something using GROUP BY and HAVING?
Try with:
SELECT *
FROM categories AS c
LEFT JOIN (SELECT * FROM articles ORDER BY id DESC) AS a
ON c.id = a.id_category
AND /criterias about joining/
WHERE /more criterias/
GROUP BY c.id
If you provide us with the Tables schemas, we could be a little more specific, but you could try something like (12.2.9.6. EXISTS and NOT EXISTS, SELECT Syntax for LIMIT)
SELECT *
FROM articles a
WHERE EXISTS (
SELECT 1
FROM articles
where category_id = a.category_id
AND <YourCriteria Here>
ORDER BY <Order Required : ID DESC, LastDate DESC or something?
LIMIT 1
)
Assuming the id's in the articles table represent always increasing numbers, this should work. Using the id is not semantically correct IMHO, you should actually use a time/date tamp field if one is available.
SELECT * FROM articles WHERE article_id IN
(
SELECT
MAX(article_id)
FROM
articles
WHERE [your filters here]
GROUP BY
category_id
)
Related
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;
Okay, so i have a list of posts and some posts are replies to other posts. I'd like to get a list of post parents in reverse order of replies.
I've tried group by but it always lists the wrong order and distinct is the only way i've managed to get it to work but obviously then it only lists the post id and not the rest of the data.
example of database here
The order i want to pull the posts out in is 1,3,5,4,2 These are the non-reply posts in the order of the latest reply.
SELECT DISTINCT `thread`
FROM
(
SELECT COALESCE(NULLIF(`parent_post`, 0), `postID`) AS `thread`
FROM `posts`
ORDER BY `postID` DESC
LIMIT 100
) `sub`
This pulls them out in the correct order but obviously only pulls out the postID and not the rest of the fields, i've tried group by but it loses the correct order.
A straightforward translation of your requirements to SQL would be:
select *
from posts p1
where parent_post = 0
order by (
select max("datetime")
from posts p2
where p2.parent_post = p1.postID
) desc
I.e. select all rows from posts that are thread starters (not replies) and order them by the latest timestamp from any of their replies in descending order.
It would be something like:
SELECT * FROM users ORDER BY id ORDER("abc","ghk","pqr"...);
In my order clause there might be 1000 records and all are dynamic.
A quick google search gave me below result:
SELECT * FROM users ORDER BY case id
when "abc" then 1
when "ghk" then 2
when "pqr" then 3 end;
As I said all my order clause values are dynamic. So is there any suggestion for me?
Your example isn't entirely clear, as it appears that a simple ORDER BY would suffice to order your id's alphabetically. However, it appears you are trying to create a dynamic ordering scheme that may not be alphabetical. In that case, my recommendation would be to use a lookup table for the values that you will be ordering by. This serves two purposes: first, it allows you to easily reorder the items without altering each entry in the users table, and second, it avoids (or at lest reduces) problems with typos and other issues that can occur with "magic strings."
This would look something like:
Lookup Table:
CREATE TABLE LookupValues (
Id CHAR(3) PRIMARY KEY,
Order INT
);
Query:
SELECT
u.*
FROM
users u
INNER JOIN
LookupTable l
ON
u.Id = l.Id
ORDER BY
l.Order
I know the title is confusing but its the best I could explain it. Basically im developing a cinema listings website for a company which owns two cinemas. So I have a database which has the two tables "Films" and "Listings" with data for both cinemas in them.
I'm trying to select all films and their data for one cinema if the films name shows up in the listings (since the two cinemas share all films but in the table but the may not have the same films showing)
Here is what i have come up with but I run into a problem as when the "SELECT DISTINCT" returns more than one result it obviously cant be matched with the FilmName on tbl Films.
How can i check this value for all FilmNames on tblFilms?
SELECT *
FROM tblFilms
WHERE FilmName = (SELECT DISTINCT FilmName FROM tblListings WHERE Cimema = 1)
use IN if the subquery return multiple values,
SELECT *
FROM tblFILMS
WHERE FilmName IN (SELECT DISTINCT FilmName FROM tblListings WHERE Cimema = 1)
Another way to solve thius is by using JOIN (which I recommend)
SELECT DISTINCT a.*
FROM tblFILMS a
INNER JOIN tblListings b
ON a.FilmName = b.FilmName AND
b.Cimema = 1
for faster query execution, add an INDEX on FilmName on both tables.
If you have your schemas for the tables, that would help.
That said, I believe what you want to look at is the JOIN keyword. (inner/outer/left/etc). That's exactly what JOIN is meant to do (ie your title).
I have searched high and low and have tried for hours to manipulate the various other queries that seemed to fit but I've had no joy.
I have several Tables in Microsoft SQL Server 2005 that I'm trying to join, an example of which is:
Company Table (Comp_CompanyId, Comp_Name)
GroupCode_Link Table (gcl_c_groupcodelinkid, gcl_c_groupcodeid, gcl_c_companyid)
GroupCode Table (grp_c_groupcodeid, grp_c_groupcode, grp_c_name)
ItemCode Table (itm_c_itemcodeid, itm_c_name, itm_c_itemcode, itm_c_group)
ItemCode_Link Table (icl_c_itemcodelinkid, icl_c_companyid, icl_c_groupcodeid, icl_c_itemcodeid)
I'm using Link Tables to associate a Group to a Company, and an Item to a Group, so a Company could have multiple groups, with multiple items in each group.
Now, I'm trying to create an Advanced Find Function that will allow a user to enter, for example, an Item Code and the result should display those companies that have that item, sounds nice and simple!
However, I haven't done something right, if I use the following query ' if the company has this item OR this item, display it's name', I get the company appearing twice in the result set, once for each item.
What I need is to be able to say is:
"Show me a list of companies that have these items (displaying each company only once!)"
I've had a go at using COUNT, DISTINCT and HAVING but have failed on each as my query knowledge isn't up to it!
First, from your description it sounds like you might have a problem with your E-R (entity-relationship) model. Your description tells me that your E-R model looks something like this:
Associative entities (CompanyGroup, GroupItem) exist to implement many-to-many relationships (since many-to-many isn't supported directly by relational databases).
Nothing wrong with that if a group can exist within multiple companies or an item across multiple groups. It would seem more likely that, at least, each group is specific to a company (I can see items existing across multiple companies and/or groups: more than one company retails, for instance, Cuisinart food processors). If that is the case, a better E-R model would be to make each group a dependent entity with a CompanyID that is a component of its primary key. It's a dependent entity because the group doesn't have an independent existence: it's created by/on behalf of and exists for its parent company. If the company goes away, the group(s) tied to it go away. No your E-R model looks like this:
From that, we can write the query you need:
select *
from Company c
where exists ( select *
from GroupItem gi
where gi.ItemID in ( desired-itemid-1 , ... , desired-itemid-n )
and gi.CompanyID = c.CompanyID
)
As you can see, dependent entities are a powerful thing. Because of the key propagation, queries tend to get simpler. With the original data model, the query would be somewhat more complex:
select *
from Company c
where exists ( select *
from CompanyGroup cg
join GroupItem gi on gi.GroupId = cg.GroupID
where gi.ItemID in ( desired-itemid-1 , ... , desired-itemid-n )
and cg.CompanyID = c.CompanyID
)
Cheers!
SELECT *
FROM company c
WHERE (
SELECT COUNT(DISTINCT icl_c_itemcodeid)
FROM GroupCode_Link gl
JOIN ItemCode_Link il
ON il.icl_c_groupcodeid = gcl_c_groupcodeid
WHERE gl.gcl_c_companyid = c.Comp_CompanyId
AND icl_c_companyid = c.Comp_CompanyId
AND icl_c_itemcodeid IN (#Item1, #Item2)
) >= 2
Replace >= 2 with >= 1 if you want "any item" instead of "all items".
If you need to show companies that have item1 AND item2, you can use Quassnoi's answer.
If you need to show companies that have item1 OR item2, then you can use this:
SELECT
*
FROM
company
WHERE EXISTS
(
SELECT
icl_c_itemcodeid
FROM
GroupCode_Link
INNER JOIN
ItemCode_Link
ON icl_c_groupcodeid = gcl_c_groupcodeid
AND icl_c_itemcodeid IN (#item1, #item2)
WHERE
gcl_c_companyid = company.Comp_CompanyId
AND
icl_c_companyid = company.Comp_CompanyId
)
I would write something like the code below:
SELECT
c.Comp_Name
FROM
Company AS c
WHERE
EXISTS (
SELECT
1
FROM
GroupCode_Link AS gcl
JOIN
ItemCode_Link AS icl
ON
gcl.gcl_c_groupcodeid = icl.icl_c_groupcodeid
JOIN
ItemCode AS itm
ON
icl.icl_c_itemcodeid = itm.itm_c_itemcodeid
WHERE
c.Comp_CompanyId = gcl.gcl_c_companyid
AND
itm.itm_c_itemcode IN (...) /* here provide list of one or more Item Codes to look for */
);
but I see there's a icl_c_companyid column in the ItemCode_Link so using GroupCode_Link table is not necessary?