Avoid duplicates when migrating from one table to another - tsql

Question

I need to migrate data from an old Books table:
create table dbo.Books_OLD (
Id int identity not null constraint PK_Books_OLD_Id primary key (Id),
Title nvarchar (200) not null,
Image varbinary (max) null,
Preview varbinary (max) null
)
To a new table structure:
create table dbo.Books (
Id int identity not null constraint PK_Books_Id primary key (Id),
Title nvarchar (200) not null
)
create table dbo.Files (
Id int identity not null constraint PK_Files_Id primary key (Id),
Content varbinary (max) null,
Name nvarchar (280) null
)
create table dbo.BookFiles (
BookId int not null,
FileId int not null,
constraint PK_BookFiles_Id primary key (BookId, FileId)
)
alter table dbo.BookFiles
add constraint FK_BookFiles_BookId foreign key (BookId) references Books(Id) on delete cascade on update cascade,
constraint FK_BookFiles_FileId foreign key (FileId) references Files(Id) on delete cascade on update cascade;
The migration should run as follows:
Books_OLD.Title => Create new Book with given Title value
Books_OLD.Image => Create new File with Image content.
Create new BookFile to associate File to Book.
Books_OLD.Preview => Create new File with Preview content.
Create new BookFile to associate File to Book.
I was able to migrate the data but somehow when I run this:
select FileId
from BookFiles
group by FileId
having count(*) > 1;
I have duplicates. I should not have duplicate FileIds. What am I missing?
The migration code I have is:
DECLARE #BOOKS table (
BookId int,
Image varbinary(max),
Preview varbinary(max)
)
MERGE Books AS d
USING Books_OLD AS s
ON 0 = 1
WHEN NOT MATCHED
THEN INSERT (Title)
VALUES (s.Title)
OUTPUT INSERTED.Id, s.Image, s.Preview
INTO #BOOKS;
INSERT Files (Content, Created)
SELECT t.Content, GETUTCDATE()
FROM #BOOKS i
CROSS APPLY (VALUES (Preview, 'Preview'), (Image, 'Image')) t(Content, ContentType)
WHERE Content IS NOT NULL
INSERT BookFiles (BookId, FileId)
SELECT i.BookId, f.Id
FROM #BOOKS i
JOIN Files f
ON f.Content = i.Image
UNION ALL
SELECT i.BookId, f.Id
FROM #BOOKS i
JOIN Files f
ON f.Content = i.Preview
Some Books can have two files (Image and Preview) so BookId can appear more than once in BooksFiles.
But each file (Image or Preview) in Books_OLD table should only be associated with one Book. So it is strange that I have duplicated FileId in BookFiles.
What am I missing?

Answer 1

If you have the same image or preview for different book in your Books_Old, in your original code from this part:
INSERT BookFiles (BookId, FileId)
SELECT i.BookId, f.Id
FROM #BOOKS i
JOIN Files f
ON f.Content = i.Image
It will return you more results when doing the INNER JOIN because two image or preview from different books can be joined. And the duplicate FileId is actually a bad record, because the BookId is not correspond to that particular Image or Preview even though they are the same.
What you could do is have another table variable called #Files, similar to the Files table structure, you just need to add one more column, which is BookId, then:
INSERT BookFiles (BookId, FileId)
SELECT i.BookId, f.Id
FROM #BOOKS i
JOIN #Files f
ON f.Content = i.Image
AND f.BookId = i.BookId --added joining condition
--assume code before has inserted bookId into `#Files`
So at last, you pick all the needed columns from #Files, insert them to Files.
UPDATE: please refer below for the full codes:
DECLARE #BOOKS table (
BookId int,
Image varbinary(max),
Preview varbinary(max)
)
--Added #File Variable
DECLARE #Files table
(
BookId int,
Content varbinary (max) null,
Created nvarchar (280) null,
Id int identity(1,1) not null primary key
)
MERGE Books AS d
USING Books_OLD AS s
ON 0 = 1
WHEN NOT MATCHED
THEN INSERT (Title)
VALUES (s.Title)
OUTPUT INSERTED.Id, s.Image, s.Preview
INTO #BOOKS;
INSERT #Files (BookId,Content, Created) --
SELECT i.BookId,t.Content, GETUTCDATE()
FROM #BOOKS i
CROSS APPLY (VALUES (Preview, 'Preview'), (Image, 'Image')) t(Content, ContentType)
WHERE Content IS NOT NULL
INSERT BookFiles (BookId, FileId)
SELECT i.BookId, f.Id
FROM #BOOKS i
JOIN #Files f
ON f.Content = i.[Image]
AND f.BookId = i.BookId --added joining condition
UNION ALL
SELECT i.BookId, f.Id
FROM #BOOKS i
JOIN #Files f
ON f.Content = i.Preview
AND f.BookId = i.BookId --added joining condition
--Last insert all needed from #File into File
INSERT INTO Files (Content, Created)
SELECT content,Created
FROM #Files
PS: Not sure whether there is a typo for dbo.File, you have Name in your table definition, but when inserting, its Created