Question

Related

Related to my other question:

Comment system design

Data Design

Let’s say I have a tags table:

tblTags
-------------
TagID (int)
Name (string)

And two content tables:

tblBlogs
-------------
Anchor (GUID, Primary Key)
BlogTitle (string)
+ More custom fields

tblTutorials
-------------
Anchor (GUID, Primary Key)
TutorialTitle (string)
+ More custom fields

There will be more tables with anchors as well, it’s not just 2.

Then to associate a tag with an above entity:

tblTagAnchors
-------------
TagID (int, Foreign Key)
Anchor (GUID, Foreign Key)

My question is, once I have built up my associations of blogs and tutorials with a specific tag, is there any way to write a query to return blogs or tutorials with a specific tag? Without needing to have separate queries for both the Blogs and Tutorials?

The main use would be for search, something along the lines of (pseudo):

select from tblBlogs and tblTutorials where the GUID exists in tblTagAnchors where tagID = 5

for each record returned
    if record from Blog
        response.write("<a href=blogView.aspx?ID=" + recID)
    else if record from Tutorial
        response.write("<a href=tutorialView.aspx?ID=" + recID)
next

I’m using SQL Server 2008 Express, and ASP.net 4 (c#) if it makes much difference with Linq to SQL, but a design based answer is all I need, not any code necesserially unless for demonstration.

Is the only way to do this with multiple queries?

Answer 1

I would recommend the use of a union. This query would only have one result set but under the hood it is merging to queries together.

Select b.rec_id,'Blog' as type from tblBlogs b
inner join tblTagAnchors ta on ta.anchor = b.anchor
where ta.tagid = 5
union
Select t.rec_id,'Tutorials' as type from tblTutorials t
inner join tblTagAnchors ta on ta.anchor = t.anchor
where ta.tagid = 5

Then in your vb code just do an if on the type field. In linq you will have to write the query using .union command. I just wrote a generic sql solution. It can easily be transformed to linq though.

As a side note conditional foreign keys makes me want to shutter. In modern database design you should always avoid a key that can go to multiple tables it is hard to enforce CRUD and more confusing to query. I would suggest creating a super type of tblBlogs and tblTutorials like tblWebsites and making the key go to the supertype.