Merging two parent > child table sets

Question

I need to get the data in two parent > child table sets merged/combined into a third parent > child table.

The tables look like this:

The only difference in the three sets of tables is that TableC has a TableType column to help discern the difference between a TableA record and a TableB record.

My first thought was to use a cursor.. Here's code to create the table structure, insert some records, and then merge the data together. It works very well, sooooo....

--Create the tables

CREATE TABLE TableA
(
    ID int not null identity primary key,
    Name VARCHAR(30)
);

CREATE TABLE TableAChild
(
    ID int not null identity primary key,
    Parent int not null,
    Name VARCHAR(30),
    CONSTRAINT FK_A FOREIGN KEY (Parent) REFERENCES TableA(ID)
);

CREATE TABLE TableB
(
    ID int not null identity primary key,
    Name VARCHAR(30)
);

CREATE TABLE TableBChild
(
    ID int not null identity primary key,
    Parent int not null,
    Name VARCHAR(30),
    CONSTRAINT FK_B FOREIGN KEY (Parent) REFERENCES TableB(ID)
);

CREATE TABLE TableC
(
    ID int not null identity primary key,
    TableType VARCHAR(1),
    Name VARCHAR(30)
);

CREATE TABLE TableCChild
(
    ID int not null identity primary key,
    Parent int not null,
    Name VARCHAR(30),
    CONSTRAINT FK_C FOREIGN KEY (Parent) REFERENCES TableC(ID)
);

-- Insert some test records.. 

INSERT INTO TableA (Name) Values ('A1')
INSERT INTO TableAChild (Name, Parent) VALUES ('A1Child', SCOPE_IDENTITY())
INSERT INTO TableB (Name) Values ('B1')
INSERT INTO TableBChild (Name, Parent) VALUES ('B1Child', SCOPE_IDENTITY())

-- Needed throughout.. 
DECLARE @ID INT

-- Merge TableA and TableAChild into TableC and TableCChild
DECLARE TableACursor CURSOR
    -- Get the primary key from TableA
    FOR SELECT ID FROM TableA
OPEN TableACursor
    FETCH NEXT FROM TableACursor INTO @ID

    WHILE @@FETCH_STATUS = 0
    BEGIN
        -- INSERT INTO SELECT the parent record into TableC, being sure to specify a TableType
        INSERT INTO TableC (Name, TableType) SELECT Name, 'A' FROM TableA WHERE ID = @ID

        -- INSERT INTO SELECT the child record into TableCChild using the parent ID of the last row inserted (SCOPE_IDENTITY())
        -- and the current record from the cursor (@ID).
        INSERT INTO TableCChild(Name, Parent) SELECT Name, SCOPE_IDENTITY() FROM TableAChild WHERE Parent = @ID

        FETCH NEXT FROM TableACursor INTO @ID
    END;

CLOSE TableACursor
DEALLOCATE TableACursor

-- Repeat for TableB
DECLARE TableBCursor CURSOR
    FOR SELECT ID FROM TableB
OPEN TableBCursor
    FETCH NEXT FROM TableBCursor INTO @ID

    WHILE @@FETCH_STATUS = 0
    BEGIN
        INSERT INTO TableC (Name, TableType) SELECT Name, 'B' FROM TableB WHERE ID = @ID
        INSERT INTO TableCChild(Name, Parent) SELECT Name, SCOPE_IDENTITY() FROM TableBChild WHERE Parent = @ID
        FETCH NEXT FROM TableBCursor INTO @ID
    END;

CLOSE TableBCursor
DEALLOCATE TableBCursor

Now, my question(s):

• I've always been told that cursors are bad. But I couldn't find another way of doing it. I'm wondering if there's some way to do that with a CTE?
• If the cursor is appropriate in this situation, how did I do? Is there a better way of doing what I did? It doesn't look very DRY to me, but I'm no SQL expert.

Lastly, if you want to re-run the query above, here's a small script to delete the tables that were created.

DROP TABLE TableAChild
DROP TABLE TableBChild
DROP TABLE TableCChild

DROP TABLE TableA
DROP TABLE TableB
DROP TABLE TableC

The correct result should look like:

Answer 1

You can use merge as described by Adam Machanic in Dr. OUTPUT or: How I Learned to Stop Worrying and Love the MERGE and in this question to get a mapping between the new identity value and the old primary key value in a table variable and the use that when you insert to your child tables.

declare @T table(ID int, IDC int);

merge dbo.TableC as C
using dbo.TableA as A
on 0 = 1
when not matched by target then
  insert (TableType, Name) values('A', A.Name)
output A.ID, inserted.ID into @T(ID, IDC);

insert into dbo.TableCChild(Parent, Name)
select T.IDC, AC.Name
from dbo.TableAChild as AC
  inner join @T as T
    on AC.Parent = T.ID;

delete from @T;

merge dbo.TableC as C
using dbo.TableB as B
on 0 = 1
when not matched by target then
  insert (TableType, Name) values('B', B.Name)
output B.ID, inserted.ID into @T(ID, IDC);

insert into dbo.TableCChild(Parent, Name)
select T.IDC, BC.Name
from dbo.TableBChild as BC
  inner join @T as T
    on BC.Parent = T.ID;

SQL Fiddle

Answer 2

Here is one way to do this without a cursor or other RBAR type stuff.

ALTER TABLE TableC ADD LegacyID INT
GO

INSERT INTO TableC (TableType, Name, LegacyID)
SELECT 'A', Name, ID
FROM TableA

INSERT TableCChild
SELECT C.ID, AC.Name
FROM TableAChild AC
JOIN TableA A ON A.Id = AC.ID
JOIN TableC C ON C.LegacyID = A.ID AND C.TableType = 'A'

INSERT INTO TableC (TableType, Name, LegacyID)
SELECT 'B', Name, ID
FROM TableB

INSERT TableCChild
SELECT C.ID, AC.Name
FROM TableBChild AC
JOIN TableB A ON A.Id = AC.ID
JOIN TableC C ON C.LegacyID = A.ID AND C.TableType = 'B'

ALTER TABLE TableC DROP COLUMN LegacyID
GO