Question

Disclaimer: my SQL skills are basic, to say the least.

Let’s say I have two similar data types in different tables of the same database.

The first table is called hardback and the fields are as follows:

hbID | hbTitle | hbPublisherID | hbPublishDate

The second table is called paperback and its fields hold similar data but the fields are named differently:

pbID | pbTitle | pbPublisherID | pbPublishDate

I need to retrieve the 10 most recent hardback and paperback books, where the publisher ID is 7.

This is what I have so far:

SELECT TOP 10
    hbID, hbTitle, hbPublisherID, hbPublishDate AS pDate
    bpID, pbTitle, bpPublisherID, pbPublishDate AS pDate
FROM hardback CROSS JOIN paperback
WHERE (hbPublisherID = 7) OR (pbPublisherID = 7)
ORDER BY pDate DESC

This returns seven columns per row, at least three of which may or may not be for the wrong publisher. Possibly four, depending on the contents of pDate, which is almost certainly going to be a problem if the other six columns are for the correct publisher!

In an effort to release an earlier version of this software, I ran two separate queries fetching 10 records each, then sorted them by date and discarded the bottom ten, but I just know there must be a more elegant way to do it!

Any suggestions?

Aside: I was reviewing what I’d written here, when my Mac suddenly experienced a kernel panic. Restarted, reopened my tabs and everything I’d typed was still here! Stack Exchange sites are awesome 🙂

Answer 1

The easiest way is probably a UNION:

SELECT TOP 10 * FROM
(SELECT hbID, hbTitle, hbPublisherID as PublisherID, hbPublishDate as pDate
 FROM hardback
 UNION 
 SELECT hpID, hpTitle, hpPublisher, hpPublishDate
 FROM paperback
) books
WHERE PublisherID = 7

If you could have two copies of the same title (1 paperback, 1 hardcover), change the UNION to a UNION ALL; UNION alone discards duplicates. You could also add a column that indicates what book type it is by adding a pseudo-column to each select (after the publish date, for instance):

hbPublishDate as pDate, 'H' as Covertype

You’ll have to add the same new column to the paperback half of the query, using ‘P’ instead. Note that on the second query you don’t have to specify column names; the resultset takes the names from the first one. All column data types in the two queries have match, also – you can’t UNION a date column in the first with a numeric column in the second without converting the two columns to the same datatype in the query.

Here’s a sample script for creating two tables and doing the select above. It works just fine in SQL Server Management Studio.Just remember to drop the two tables (using DROP Table tablename) when you’re done.

use tempdb;
create table Paperback (pbID Integer Identity, 
    pbTitle nvarchar(30), pbPublisherID Integer, pbPubDate Date);
create table Hardback (hbID Integer Identity, 
    hbTitle nvarchar(30), hbPublisherID Integer, hbPubDate Date);

insert into Paperback (pbTitle, pbPublisherID, pbPubDate)
  values ('Test title 1', 1, GETDATE());
insert into Hardback (hbTitle, hbPublisherID, hbPubDate)
  values ('Test title 1', 1, GETDATE());

select * from (
  select pbID, pbTitle, pbPublisherID, pbPubDate, 'P' as Covertype
  from Paperback
  union all
  select hbID, hbTitle, hbPublisherID, hbPubDate,'H' 
  from Hardback) books
order by CoverType;

/* You'd drop the two tables here with
DROP table Paperback;
DROP table HardBack;
*/