SELECT CASE/JOIN 无行时

Question

Sorry this one is a bit of a headache. I'll start with the example:

Tables:

TownCountry

Record |  Town  | CountryCode
-------+--------+-------------
1      | London | A1
2      | Cardiff| A2
3      | Hull   | A1
4      | Luton  | A1

ReFData

Type    |  Code   | Country
--------+---------+-------------
Country | A1      | England
Country | A2      | Wales

If my query is:

select a.Town, b.Country from TownCountry a, RefData b, TownCountry c
where a.Record=1
and b.Code=c.CountryCode and c.Record=2

I get:

London | Wales

However, if I change the code for Wales to A3, and keep the query the same, by result returns no rows.

What I want, in the example where Wales is A3, is for my result to be:

London | (empty)

I've tried COALESCE:

select a.Town, COALESCE(b.Country,'empty') from TownCountry a, RefData b, TownCountry c
where a.Record=1
and b.Code=c.CountryCode and c.Record=2

but this returned no rows

I also tried select case, right and left joins, but still no rows.

Here's a simpler example that my good friend just gave me while discussing:

Towns

Record |  Town  
-------+--------
1      | London 
2      | Cardiff
4      | Luton

select a.Town, b.Town, c.town, d.Town
from Towns a, Towns b, Towns c, Towns d
where a.Reocrd=1 and b.Reocrd=2 and c.Reocrd=3 and a.Reocrd=4

I want to return

a.Town | b.Town | c.Town | d.Town
-------+--------+--------+--------
London | Cardiff| NULL   | Luton

Any help much appreciated.

Answer 1

If you want to keep rows even where there is no match on the column you're joining on, you need to do an OUTER JOIN rather than an INNER JOIN .

http://publib.boulder.ibm.com/infocenter/db2luw/v9r7/index.jsp?topic=/com.ibm.db2.luw.apdv.porting.doc/doc/r0052878.html

Answer 2

You're not really doing joins, and you need an outer join (ie LEFT JOIN ).

What you want is something like this:

select a.Town, b.Country
from TownCountry a
left join RefData b on b.Code = a.CountryCode
left join TownCountry c on c.CountryCode = b.Code and c.Record=2
where a.Record=1;

EDITED : I put "and c.Record=2" into the join clause. This little trick is a good one - it retains the condition, but doesn't require a joined row

Answer 3

The problem here is that the Translation table does not have entry for blank raw values. As a result, there is nothing in the Translation table that matches so no rows are returned.

This particular problem can be solved by adding a row to the Translation table, or more precisely, using union to add the row:

select a.Town, b.Country from TownCountry a, 
(select Code, Country from RefData b
union select '' as Code, 'Not found' as Country from RefData c), TownCountry c
where a.Record=1
and b.Code=c.CountryCode and c.Record=2

SQL Love, Wing