I have three tables:
COLLECTION
PERSON
PERSON_COLLECTION
where PERSON_COLLECTION is a mapping table id|person_id|collection_id
I now want to select all entries in collection and order them by person.name.
Do I have to join the separate tables with the mapping table first and then do a join again on the results?
SELECT
c.*,
p.Name
FROM
Collection c
JOIN Person_Collection pc ON pc.collection_id = c.id
JOIN Person p ON p.id = pc.person_id
ORDER BY p.Name
Not sure without the table schema but, my take is:
SELECT
c.*,
p.*
FROM
Person_Collection pc
LEFT JOIN Collection c
ON pc.collection_id = c.id
LEFT JOIN Person p
ON pc.person_id = p.id
ORDER BY p.name
The order you join won't break it but depending on which sql product you're using may effect performance.
You need to decide if you want ALL records from both/either table or only records which have a matching mapping entry, this will change the type of join you need to use.
Related
I am working on a query that has fields called ios_app_id, android_app_id and app_id.
The app_id from downloads table can be either ios_app_id or android_app_id in the products table.
Is it correct that because of that I cannot just run a simple join of downloads and products table on on p.ios_app_id = d.app_id and then join again on on p.android_app_id = d.app_id? Would that cause an incorrect number of records?
select p.product_id, d.date, d.downloads,
from products p
inner join download d
on p.ios_app_id = d.app_id
UNION
select p.product_id, d.date, d.downloads
from products p
inner join download d
on p.android_app_id = d.app_id
I would try:
select p.product_id, d.date, d.downloads,
from products p
inner join downloads d
on p.ios_app_id = d.app_id
inner join downloads d
on p.android_app_id = d.app_id
Basically I am trying to understand why the union here is needed instead of just joining the two fields twice? Thank you
Just join twice:
select p.product_id,
coalesce(di.date, da.date),
coalesce(di.downloads, da.downloads)
from products p left join
downloads di
on p.ios_app_id = di.app_id left join
downloads da
on p.android_app_id = da.app_id;
This should be more efficient than your method with union. Basically, it attempts joining using the two ids. The coalesce() combines the results into a single column.
Remember that the purpose of an INNER JOIN is to get the values that exists on BOTH sets of data (lets called them table A and table B), using a specific column to join them. In your example, if you try to do the INNER JOIN twice, what would happen is that the first time you execute the INNER JOIN, the complete PRODUCTS table is your table A, and you obtain all the products that have downloaded the ios_app, but now (and this is the key part) this result becomes your new dataset, so it becomes your new table A for the next inner join. And thats the issue, cause what you would want is to join the whole table again, not just the result of the first join, but thats not how it works. This is why you need to use the UNION, cause you need to obtain your results independently and then add them.
An alternative would be to use LEFT JOIN, but you could get null values and duplicates -and its not too "clean"-. So, for your particular case, I think using UNION is much clearer and easier to understand.
If you do left join in first query it will work.
create table all_products as (select p.product_id, d.date, d.downloads,
from products p
left join downloads d
on p.ios_app_id = d.app_id)
select a.product_id, d.date, d.downloads from all_products a left join downloads d
on a.android_app_id = d.app_id inner join
Here is my ERD for SQL Server:
https://c2.staticflickr.com/6/5832/23786188186_d6f1d93132_o.jpg
I need to find out which books are associated with each publisher.
USE BookStoreDB
SELECT ProductID
FROM Books
INNER JOIN [Publishers] PublisherID ON PublishersID = ProductID
I'm assuming I just didn't create the INNER JOIN command correctly?
SELECT p.PublisherID, b.ProductID
FROM Books b
INNER JOIN Publishers p ON p.PublisherID = b.PublisherID
The ON part of a join defines how the two tables are related to each other. In your case, it would be:
INNER JOIN Publishers ON (Books.PublisherID = Publishers.PublisherID)
If you put something between the name of the table, and the ON, it's treated as an alias. You can (and should) also use these aliases in your SELECT
FROM Books B
INNER JOIN Publishers P ON (B.PublisherID = P.PublisherID)
Finally, you probably want to select information that will actually tell you who the publisher is:
SELECT B.ISBN, P.CompanyName
FROM Books B
INNER JOIN Publishers P ON (B.PublisherID = P.PublisherID)
There Is three table Master, Regular and Customer.
I'm saving ControlId in customer master for both Master and Regular. I want to get Profile from Master from the Customer Record.
By using below Query. I'm able to get MasterID from regular But I want Profile.
Query
select * from customer where refId='R000003'
(select ControlId from regular where LicenseId='R000003')
Result
Master Table
Regular Table
My Query Is..
SELECT Customer.CustomerId, Regular.LicenseId, Regular.ControlId,
Master.FullName, Master.profile
FROM Customer INNER JOIN
Regular ON Customer.RefId = Regular.LicenseId INNER JOIN
Master ON Regular.ControlId = Master.MasterId
WHERE (Customer.RefId = 'R000003')
But Its showing Regural's only I want Masters record also...
Is this what you mean? I'm not sure..
select regular.ControlId, master.profile
from regular r inner join master m ON (r.controlId = m.masterId)
where regular.LicenseId='R000003'
Posting an image of your data is not helpful. No one is going to type this in.
Paste the sample.
I am going to guess that RefId and LicenseId refer to each other. I think this is the query you want:
select c.*, m.profile
from customer c join
regular r
on c.refId = r.LicenseId join
master m
on r.controlId = m.MasterId;
I would advise you to fix your tables. Join keys in different tables should have similar names, so you know they line up. In fact, I almost always name my join keys as "Id", so this query would look more like like:
select c.*, m.profile
from customer c join
regular r
on c.CustomerId = r.CustomerId join
master m
on r.MasterId = m.MasterId;
I already read a lot on that topic but I´m unable to get it to work for my case.
I have the following situation:
A list of orderitems (the main datasets I want to get)
Articles which have a 1:1 relation to an order item
A n:m Jointable "Articlesupplier" which creates a relation between an article and a
partner
A Partner table with detailed information about partners.
Target:
One dataset per OrderItem and from the suppliers I only want to get the first one found in the join. No priorization required.
Tables:
Table IDX_ORDERITEM
id,article_id
Table IDX_ARTICLE
id,name
Table IDX_ARTICLESUPPLIER
article_id,partner_id
Table IDX_PARTNER
id,abbr
My actual statement (short version):
SELECT IDX_ORDERITEM.id
FROM
dbo.IDX_ORDERITEM AS IDX_ORDERITEM
-- ARTICLE --
INNER JOIN dbo.IDX_ARTICLE AS IDX_ARTICLE
ON IDX_ORDERITEM.article_id=IDX_ARTICLE.id
-- SUPPLIER VIA ARTICLE --
LEFT JOIN
(SELECT TOP(1) IDX_PARTNER.id, IDX_PARTNER.abbr
FROM IDX_PARTNER, IDX_ARTICLESUPPLIER
WHERE IDX_PARTNER.id = IDX_ARTICLESUPPLIER.partner_id
AND IDX_ARTICLESUPPLIER.article_id=IDX_ARTICLE.id) AS IDX_PARTNER_SUPPLIER
ON IDX_PARTNER_SUPPLIER.id=IDX_ARTICLE.supplier_partner_id
WHERE 1>0
ORDER BY orderitem.id DESC
But it seems I can´t access IDX_ARTICLE.id in the subquery. I get the following error message:
The multi-part identifier "IDX_ARTICLE.id" could not be bound.
Is the problem that the Article alias has the same name as the table name?
Thanks a lot in advance for possible ideas,
Mike
Well, I changed your aliases, and the subquery to which you were joining (I also modified that subquery so it doesn't use implicit joins anymore), though this changes where mostly cosmetics. The actual important change was the use of OUTER APPLY instead of LEFT JOIN:
SELECT OI.id
FROM dbo.IDX_ORDERITEM AS OI
INNER JOIN dbo.IDX_ARTICLE AS A
ON OI.article_id = A.id
OUTER APPLY
(SELECT TOP(1) P.id, P.abbr
FROM IDX_PARTNER AS P
INNER JOIN IDX_ARTICLESUPPLIER AS SUP
ON P.id = SUP.partner_id
WHERE SUP.article_id = A.id
AND P.id = A.supplier_partner_id) AS PS
ORDER BY OI.id DESC
The error is thrown because the below piece of query
(SELECT TOP(1) IDX_PARTNER.id, IDX_PARTNER.abbr
FROM IDX_PARTNER, IDX_ARTICLESUPPLIER
WHERE IDX_PARTNER.id = IDX_ARTICLESUPPLIER.partner_id
AND IDX_ARTICLESUPPLIER.article_id=IDX_ARTICLE.id) AS IDX_PARTNER_SUPPLIER
cannot be considered as a correlated sub-query and IDX_ARTICLE.id is referenced in it in the same manner we reference a field of outer query in a correlated sub-query.
I see two problems.
According to your DDLs there is no IDX_ARTICLE.supplier_partner_id which you refer to in the left join on clause.
Second, I'm quite sure you cannot use IDX_ARTICLE.id in your derived table. Simply add IDX_ARTICLESUPPLIER.article_id to your derived table selected fields and use it in your left join on clause against IDX_ARTICLE.id.
I prefer to avoid nested queries. If I can, I will always rewrite it using CTE.
WITH Part_Sup
AS (
SELECT TOP ( 1 ) P.id
,P.abbr
,SUP.article_id
FROM IDX_PARTNER AS P
INNER JOIN IDX_ARTICLESUPPLIER AS SUP
ON P.id = SUP.partner_id
)
SELECT OI.id
FROM dbo.IDX_ORDERITEM AS OI
INNER JOIN dbo.IDX_ARTICLE AS A
ON OI.article_id = A.id
LEFT OUTER JOIN Part_Sup AS PS
ON PS.article_id = A.Id
AND PS.id = A.supplier_partner_id
ORDER BY OI.id DESC;
Next I rewritten the first query to use ROW_NUMBER() function instead of using TOP (1) using ROW_NUMBER you can control which results you want and what you don't want.
WITH Part_Sup
AS (
SELECT P.id
,P.abbr
,SUP.article_id
,ROW_NUMBER() OVER ( PARTITION BY P.id, P.abbr ) AS RowNum
FROM IDX_PARTNER AS P
INNER JOIN IDX_ARTICLESUPPLIER AS SUP
ON P.id = SUP.partner_id
)
SELECT OI.id
FROM dbo.IDX_ORDERITEM AS OI
INNER JOIN dbo.IDX_ARTICLE AS A
ON OI.article_id = A.id
LEFT OUTER JOIN Part_Sup AS PS
ON PS.article_id = A.Id
AND PS.id = A.supplier_partner_id
AND RowNum = 1
ORDER BY OI.id DESC;
Thanks Lamak - you solved it :)
I used your input to extract the basic solution to make it a bit easier to read for others which have the same problem:
Using OUTER APPLY (without ORDER_ITEM Table here):
SELECT IDX_ARTICLE.id AS AR_ID, IDX_PARTNER_SUPPLIER.id, IDX_PARTNER_SUPPLIER.abbr
FROM
dbo.IDX_ARTICLE AS IDX_ARTICLE
OUTER APPLY
(SELECT TOP(1) _PARTNER.id, _PARTNER.abbr
FROM IDX_PARTNER AS _PARTNER
INNER JOIN IDX_ARTICLESUPPLIER AS _ARTICLESUPPLIER
ON _PARTNER.id = _ARTICLESUPPLIER.partner_id
WHERE _ARTICLESUPPLIER.article_id=IDX_ARTICLE.id
AND _ARTICLESUPPLIER.deleted IS NULL) AS IDX_PARTNER_SUPPLIER
WHERE IDX_ARTICLE.id=67
I have three table in the Database -
Activity table with activity_id, activity_type
Category table with category_id, category_name
Link table with mapping between activity_id and category_id
I need to write a select statement to get the following data:
activity_id, activity_type, Category_name.
The issue is some of the activity_id have no entry in the link table.
If I write:
select a.activity_id, a.activity_type, c.category_name
from activity a, category c, link l
where a.activity_id = l.activity_id and c.category_id = l.category_id
then I do not get the data for the activity_ids that are not present in the link table.
I need to get data for all the activities with empty or null value as category_name for those not having any linking for category_id.
Please help me with it.
PS. I am using MS SQL Server DB
I believe you're looking for a LEFT OUTER JOIN for your activity table to return all rows.
SELECT
a.activity_id, a.activity_type, c.category_name
FROM activity a
LEFT OUTER JOIN link l
ON a.activity_id = l.activity_id
LEFT OUTER JOIN category c
ON c.category_id = l.category_id;
You should use proper explicit joins:
select a.activity_id, a.activity_type, c.category_name
from activity a
LEFT JOIN link l
ON a.activity_id = l.activity_id
LEFT JOIN category c
ON l.category_id = c.category_id
If writing this type of logic will be part of your ongoing responsibilities, I would strongly suggest that you do some research on joins, including the interactions between joins and where clauses. Joins and where clauses combine to form the backbone of query writing, regardless of the technology used to retrieve the data.
Most critical join information to understand:
Left Outer Join: retrieves all information from the 'left' table and any records that exist in the joined table
Inner Join: retrieves only records that exist in both tables
Where clauses: used to limit data, regardless of inner or outer join definitions.
In the example you posted, the where clause is limiting your overall data to rows that exist in all 3 tables. Replacing the where clause with appropriate join logic will do the trick:
select a.activity_id, a.activity_type, c.category_name
from activity a
left outer join link l --return all activity rows regardless of whether the link exists
on a.activity_id = l.activity_id
left outer join category c --return all activity rows regardless of whether the link exists
on c.category_id = l.category_id
Best of luck!
What about?
select a.activity_id, a.activity_type, c.category_name from category c
left join link l on c.category_id = l.category_id
left join activity a on l.activity_id = a.activity_id
Actually, the first join seems that it could be an inner join, because you didn't mention that there might be some missing elements there