How to show one to many relationship between two tables in SQL? - sql

Question

I have two tables A and B.
A table contain
postid,postname,CategoryURl
and
B table contain
postid,CategoryImageURL
For one postid there are multiple CategoryImageURL assigned.I want to display that CategoryImageURL in Table A but for one postid there should be CategoryImageURL1,CategoryImageURL2 should be like that one.
I want to achieve one to many relationship for one postid then what logic should be return in sql function??

Answer 1

In my eyes it seems that you want to display all related CategoryImageURLs of the second table in one line with a separator in this case the comma?
Then you will need a recursive operation there. Maybe a CTE (Common Table Expression) does the trick. See below. I have added another key to the second table, to be able to check, if all rows of the second table have been processed for the corresponding row in the first table.
Maybe this helps:
with a_cte (post_id, url_id, name, list, rrank) as
(
select
a.post_id
, b.url_id
, a.name
, cast(b.urln + ', ' as nvarchar(100)) as list
, 0 as rrank
from
dbo.a
join dbo.b
on a.post_id = b.post_id
union all
select
c.post_id
, a1.url_id
, c.name
, cast(c.list + case when rrank = 0 then '' else ', ' end + a1.urln as nvarchar(100))
, c.rrank + 1
from a_cte c
join ( select
b.post_id
, b.url_id
, a.name
, b.urln
from dbo.a
join dbo.b
on a.post_id = b.post_id
) a1
on c.post_id = a1.post_id
and c.url_id < a1.url_id -- ==> take care, that there is no endless loop
)
select d.name, d.list
from
(
select name, list, rank() over (partition by post_id order by rrank desc)
from a_cte
) d (name, list, rank)
where rank = 1

Answer 2

You are asking the wrong sort of question. This is about normalization.
As it stands, you have a redundancy? Where each postname and categoryURL is represented by an ID field.
For whatever reason, the tables separated CategoryImageUrl into its own table and linked this to each set of postname and categoryURL.
If the relation is actually one id to each postname, then you can denormalize the table by adding the column CategoryImageUrl to your first table.
Postid, postname, CategoryURL, CategoryImageUrl
Or if you wish to keep the normalization, combine like fields into their own table like so:
--TableA:
Postid, postname, <any other field dependent on postname >
--TableA
Postid, CategoryURL, CategoryImageUrl
Now this groups CategoryURL together but uses a redundancy of having multiple CategoryURL to exist. However, Postid has only one CategoryUrl.
To remove this redundancy in our table, we could use a Star Schema strategy like this:
-- Post Table
Postid, postname
-- Category table
CategoryID, CategoryURL, <any other info dependent only on CategoryURL>
-- Fact Table
Postid, CategoryID, CategoryImageURL
DISCLAIMER: Naturally I assumed aspects of your data and might be off. However, the strategy of normalization is still the same.
Also, remember that SQL is relational and deals with sets of data. Inheritance is incompatible to the relational set theory. Every table can be queried forwards and backwards much the way every page and chapter in a book is treated as part of the book. At no point would we see a chapter independent of a book.