联接表以从一个或另一个表中选择数据的更好方法

Question

BookID  Title           ReleaseYear
1       The Hobbit      1937
2       Atlas Shrugged  1957

BookID  Cost    BookPrinterID
1       12      38

BookID  Charge  BookPublisherID
1       39      148
2       45      151

That's the book publishing tables data I have.
Books table, Cost table and Charge table.
I would like to see cost information if present else charge data from single query. ie

BookID  Cost
1       12
2       45

This is my query

select Books.BookID, ISNULL(Cost.Cost, Charge.Charge) AS Cost 
from Books
left join Cost on Books.BookID = Cost.BookID
left join Charge on Book.BookID = Charge.BookID

That works, but problem is if there are more tables to join or more columns to retrieve, having ISNULL condition for every column will become a big blob of text, especially if you have to parse xml.

ISNULL(Cost.Xchange.value('/Partner[1]/@Sales[1]', 'nvarchar(500)'), Charge.Xchange.value('/Partner[1]/@Sales[1]', 'nvarchar(500)')) AS Xchange

My question is, is there a neater way to write this query?

Answer 1

Using an INNER join on both tables, will omit the row for bookid=2 as there is no record in the cost table for it. You likely want to use LEFT OUTER JOIN instead of INNER JOIN for both tables to show all rows.

That said, what you want is some sort of intermediate table that is a combination of cost / charge but only when there is no record in cost .

You could do something like this, where you populate a new table with all the cost records, and then all the charge records (where no cost is found in the new table)... Although, there would undoubtedly be a performance hit by creating/writing to a new table on the fly. I wouldn't want to do this in a large library.

Here's a sql fiddle too.

create table #books (bookid int, title varchar(50), releaseyear int)
create table #charge (bookid int, charge money, publisherid int)
create table #cost  (bookid int, cost money, bookprinterid int)

insert into #books 
select 1, 'The Hobbit', 1937 union
select 2, 'Atlas Shrugged', 1957

insert into #cost
select 1, 12, 38

insert into #charge
select 1, 39, 148 union
select 2, 45, 151

create table #allcosts (bookid int, cost money)

insert into #allcosts
select c1.bookid, c1.cost from #cost c1

insert into #allcosts
select c2.bookid, c2.charge 
from #charge c2
left outer join #allcosts c3 on c2.bookid=c3.bookid
where c3.bookid is null

select b.BookID, c.cost AS Cost 
from #Books b
inner join #allcosts c on b.bookid = c.bookid

Answer 2

If all rows in Books are not in Cost you can't use inner joins on the 3 tables. Otherwise in your example BookID=2 will never be returned. There is no way around using isNull if you are going to join more tables and retrieve more columns.

select bookid, isNull(cost, charge)
from Books b
inner join Charge c
on bt.bookid = c.bookid
left join Cost co
on co.bookid = bt.bookid