Question

I just need some confirmation is database designed like this is fine or not. And if not am I doing something wrong here.
I have following tables:

TableA{TableAID,...}
TableB{TableBID,...}
TableC{TableCID,...}
etc.

And I have one table that I use like some kind of ‘news feed’. When I add something in any table A,B,C I also add row in this table.

Feed{FeedID, TypeID, ReferenceID,...}

FeedID is PK auto increment
TypeID is number that reference types table and based on this ID I know is row in this table from table A,B,C.
ReferenceId is ID of item in tables A,B,C.
A,B,C tables all have different fields.
Now when I want to get feed data I also need to grab some data from each of this table to use it in application. In my query to get this I use a lot SELECT CASE CLAUSE like:

I first join to all tables in query (A,B,C)

...
CASE Feed.TypeId              
               WHEN 1 THEN tableA.someData
                           WHEN 2 THEN tableB.someData
                           WHEN 3 THEN tableC.someData
          END AS Data,
...

Answer 1

Without getting into suitability of this for a specific purpose, your supertype-subtype model is “reversed”.

So DDL looks something like

CREATE TABLE Feed ( 
      FeedID               integer  IDENTITY(1,1) not null
    , FeedType             char(1)  not null
  --  Common_Columns_Here
    , Common_Column        varchar(20)
);
ALTER TABLE Feed ADD CONSTRAINT pk_Feed PRIMARY KEY (FeedID) ;


CREATE TABLE Feed_A ( 
      FeedID               integer  not null
  --  A_Specific_Columns_Here
    , A_Specific_Column    varchar(20)
);
ALTER TABLE Feed_A ADD
  CONSTRAINT  pk_Feed_A PRIMARY KEY (FeedID)
, CONSTRAINT fk1_Feed_A FOREIGN KEY (FeedID) REFERENCES Feed(FeedID) ;


CREATE TABLE Feed_B (
      FeedID               integer  not null
  --  B_Specific_Columns_Here
    , B_Specific_Column    varchar(20)
);
ALTER TABLE Feed_B ADD 
  CONSTRAINT  pk_Feed_B PRIMARY KEY (FeedID)
, CONSTRAINT fk1_Feed_B FOREIGN KEY (FeedID) REFERENCES Feed(FeedID) ;


CREATE TABLE Feed_C ( 
      FeedID               integer  not null
  --  C_Specific_Columns_Here
    , C_Specific_Column    varchar(20)
);
ALTER TABLE Feed_C ADD 
  CONSTRAINT  pk_Feed_C PRIMARY KEY (FeedID)
, CONSTRAINT fk1_Feed_C FOREIGN KEY (FeedID) REFERENCES Feed(FeedID) ;

Now, in order to read from this structure, create a view first

create view vFeed as
select
      f.FeedID
    , FeedType
    , Common_Column
    , A_Specific_Column
    , B_Specific_Column
    , C_Specific_Column
from      Feed   as f
left join Feed_A as a on (a.FeedID = f.FeedID and f.FeedType = 'A')
left join Feed_B as b on (b.FeedID = f.FeedID and f.FeedType = 'B')
left join Feed_C as c on (c.FeedID = f.FeedID and f.FeedType = 'C')
;

Look what happens when I want to select data which I know is from feed A. Note that FeedType is not specified in this query, only column name which belongs to Feed_A (and common column).

select
      FeedID
    , Common_Column
    , A_Specific_Column
from vFeed;

Notice that execution plan shows only Feed and Feed_A tables, query optimizer eliminated tables _B and _C; no need to touch those two.

In other words, you can ask for a specific feed data by simply using only specific columns in a query, and let the optimizer sort everything else out — no need for CASE ... WHEN .. acrobatics from your example.