SQL join under a double condition - sql

Question

Imagine we have 2 Tables, A and B, having the same structure, as follows:
Currency, Spot, Exposure, Fixing.
We want to join these two tables in order to have a query on, for example, Fixing = '2013-01-03' and having the major list of currencies (there is not always correspondence between currencies in A and currencies in B).
1 condition: A.Fixing = B.Fixing
2 condition: A.Currency = B.Currency (returning also Currency values not matched)
This is an example: We have the USD currency in table A for that day requested but we don't have it on table B. What we want for that day is a join table having one row with USD followed by the Exposure value in Table A and followed by zero (because ARS was not in table B and therefore it has no Exposure on USD)
How can we write a query?
Below Tables and results:
Table A:
Currency Spot Exposure Fixing
-------- ------- ------------ ----------
AUD 1.3023 -504,561.00 30/01/2013
CHF 1.2378 268,243.00 30/01/2013
GBP 0.8597 2,204.00 30/01/2013
JPY 123.635 -552.00 30/01/2013
USD 1.3572 5,242.00 30/01/2013
AUD 1.300 -574,561.00 29/01/2013
CHF 1.235 545,152.00 29/01/2013
GBP 0.858 1,155,212.00 29/01/2013
JPY 123.388 -45,115.00 29/01/2013
USD 1.354 22,468.00 29/01/2013
Table B:
Currency Spot Exposure Fixing
-------- ------- ------------- ----------
AUD 1.3023 256,442.00 30/01/2013
CHF 1.2378 -4,456,421.00 30/01/2013
GBP 0.8597 15,246.00 30/01/2013
JPY 123.635 1,243,146.00 30/01/2013
AUD 1.300 41,246.00 29/01/2013
CHF 1.235 243.00 29/01/2013
GBP 0.858 54,564.00 29/01/2013
JPY 123.388 140.00 29/01/2013
Results:
Currency A.Exposure B.Exposure Fixing
-------- ----------- ------------- ----------
AUD -504,561.00 256,442.00 30/01/2013
CHF 268,243.00 -4,456,421.00 30/01/2013
GBP 2,204.00 15,246.00 30/01/2013
JPY -552.00 1,243,146.00 30/01/2013
USD 5,242.00 - 30/01/2013

Answer 1

EDITED: The left outer join allows you to grab everything in table a and matching values in table b and having NULLs where there isn't data in table b. Use the ISNULL() function to give you cleaner results by checking for NULL values and if it finds one, replacing it with a different value of your choosing.
select
a.Currency
, a.Spot
, a.Exposure as a_Exposure
, ISNULL(b.Exposure, 0) as b_Exposure
, a.Fixing
from a
left outer join b
on b.Currency = a.Currency
and b.Fixing = a.Fixing
where a.Fixing = '2013-01-30';
I have changed the query to reflect the suggestion from Clockwork-Muse. You will need to add any columns you want data returned for.