How to join three tables with distinct - sql

Question

I'm trying to join three tables to pull back a list of distinct blog posts with associated assets (images etc) but I keep coming up a cropper. The three tablets are tblBlog, tblAssetLink and tblAssets. The Blog tablet hold the blog, the asset table holds the assets and the Assetlink table links the two together.
tblBlog.BID is the PK in blog, tblAssets.AID is the PK in Assets.
This query works but pulls back multiple posts for the same record. I've tried to use select distinct and group by and even union but as my knowledge is pretty poor with SQL - they all error.
I'd like to also discount any assets that are marked as deleted (tblAssets.Deleted = true) but not hide the associated Blog post (if that's not marked as deleted). If anyone can help - it would be much appreciated! Thanks.
Here's my query so far....
SELECT dbo.tblBlog.BID,
dbo.tblBlog.DateAdded,
dbo.tblBlog.PMonthName,
dbo.tblBlog.PDay,
dbo.tblBlog.Header,
dbo.tblBlog.AddedBy,
dbo.tblBlog.PContent,
dbo.tblBlog.Category,
dbo.tblBlog.Deleted,
dbo.tblBlog.Intro,
dbo.tblBlog.Tags,
dbo.tblAssets.Name,
dbo.tblAssets.Description,
dbo.tblAssets.Location,
dbo.tblAssets.Deleted AS Expr1,
dbo.tblAssetLink.Priority
FROM dbo.tblBlog
LEFT OUTER JOIN dbo.tblAssetLink
ON dbo.tblBlog.BID = dbo.tblAssetLink.BID
LEFT OUTER JOIN dbo.tblAssets
ON dbo.tblAssetLink.AID = dbo.tblAssets.AID
WHERE ( dbo.tblBlog.Deleted = 'False' )
ORDER BY dbo.tblAssetLink.Priority, tblBlog.DateAdded DESC
EDIT
Changed the Where and the order by....
Expected output:
tblBlog.BID = 123
tblBlog.DateAdded = 12/04/2015
tblBlog.Header = This is a header
tblBlog.AddedBy = Persons name
tblBlog.PContent = *text*
tblBlog.Category = Category name
tblBlog.Deleted = False
tblBlog.Intro = *text*
tblBlog.Tags = Tag, Tag, Tag
tblAssets.Name = some.jpg
tblAssets.Description = Asset desc
tblAssets.Location = Location name
tblAssets.Priority = True

Answer 1

Use OUTER APPLY:
DECLARE #b TABLE ( BID INT )
DECLARE #a TABLE ( AID INT )
DECLARE #ba TABLE
(
BID INT ,
AID INT ,
Priority INT
)
INSERT INTO #b
VALUES ( 1 ),
( 2 )
INSERT INTO #a
VALUES ( 1 ),
( 2 ),
( 3 ),
( 4 )
INSERT INTO #ba
VALUES ( 1, 1, 1 ),
( 1, 2, 2 ),
( 2, 1, 1 ),
( 2, 2, 2 )
SELECT *
FROM #b b
OUTER APPLY ( SELECT TOP 1
a.*
FROM #ba ba
JOIN #a a ON a.AID = ba.AID
WHERE ba.BID = b.BID
ORDER BY Priority
) o
Output:
BID AID
1 1
2 1
Something like:
SELECT b.BID ,
b.DateAdded ,
b.PMonthName ,
b.PDay ,
b.Header ,
b.AddedBy ,
b.PContent ,
b.Category ,
b.Deleted ,
b.Intro ,
b.Tags ,
o.Name ,
o.Description ,
o.Location ,
o.Deleted AS Expr1 ,
o.Priority
FROM dbo.tblBlog b
OUTER APPLY ( SELECT TOP 1
a.* ,
al.Priority
FROM dbo.tblAssetLink al
JOIN dbo.tblAssets a ON al.AID = a.AID
WHERE b.BID = al.BID
ORDER BY al.Priority
) o
WHERE b.Deleted = 'False'

Answer 2

You cannot join three tables unless they all have the same attribute. It would work if all tables had BID, but the second join is trying to join AID. Which wont work. They all have to have BID.

Answer 3

Based on your comments
i would like to get is just one asset per blog post (top one ordered
by Priority)
You can change your query as following. I suggest changing the join with dbo.tblAssetLink to filtered one, which contains only one (highest priority) link for every blog.
SELECT dbo.tblBlog.BID,
dbo.tblBlog.DateAdded,
dbo.tblBlog.PMonthName,
dbo.tblBlog.PDay,
dbo.tblBlog.Header,
dbo.tblBlog.AddedBy,
dbo.tblBlog.PContent,
dbo.tblBlog.Category,
dbo.tblBlog.Deleted,
dbo.tblBlog.Intro,
dbo.tblBlog.Tags,
dbo.tblAssets.Name,
dbo.tblAssets.Description,
dbo.tblAssets.Location,
dbo.tblAssets.Deleted AS Expr1,
dbo.tblAssetLink.Priority
FROM dbo.tblBlog
LEFT OUTER JOIN
(SELECT BID, AID,
ROW_NUMBER() OVER (PARTITION BY BID ORDER BY [Priority] DESC) as N
FROM dbo.tblAssetLink) AS filteredAssetLink
ON dbo.tblBlog.BID = filteredAssetLink.BID
LEFT OUTER JOIN dbo.tblAssets
ON filteredAssetLink.AID = dbo.tblAssets.AID
WHERE dbo.tblBlog.Deleted = 'False' AND filteredAssetLink.N = 1
ORDER BY tblBlog.DateAdded DESC