O Oracle 11g e o Sql Server 2005+ contêm um comando pivot que realizará o que você deseja.
http://www.orafaq.com/wiki/PIVOT
http://msdn.microsoft.com/en-us/library/ms177410 .aspx
Caso contrário, você precisaria construir uma instrução sql dinâmica para conseguir isso.
Editar - Aqui está (versão do SQL Server).
/* Begin Set up of test data */
IF EXISTS (SELECT 1 from sys.tables WHERE name = N'Item')
DROP TABLE Item
GO
IF EXISTS (SELECT 1 from sys.tables WHERE name = N'CrossReference')
DROP TABLE CrossReference
GO
CREATE TABLE Item
(
Item varchar(20),
BasePart varchar(20),
Size varchar(20)
);
CREATE Table CrossReference
(
Item varchar(20),
CrossReferenceNumber varchar(20)
);
INSERT INTO Item VALUES ('item1', 'b1', 'Large');
INSERT INTO Item VALUES ('item2', 'bxx1', 'Large');
INSERT INTO Item VALUES ('item3', 'bddf1', 'Small');
INSERT INTO Item VALUES ('item4', 'be3f1', 'Small');
INSERT INTO Item VALUES ('item5', 'b13vx1', 'Small');
INSERT INTO CrossReference VALUES( 'item1', 'crossRef1')
INSERT INTO CrossReference VALUES('item1', 'crossRef2')
INSERT INTO CrossReference VALUES('item1', 'crossRef3')
INSERT INTO CrossReference VALUES('item1', 'crossRef4')
INSERT INTO CrossReference VALUES('item2', 'crossRef1')
INSERT INTO CrossReference VALUES('item2', 'crossRef1')
INSERT INTO CrossReference VALUES('item3', 'crossRef1')
INSERT INTO CrossReference VALUES('item4', 'crossRef2')
INSERT INTO CrossReference VALUES('item5', 'crossRef5')
INSERT INTO CrossReference VALUES('item5', 'crossRef1')
INSERT INTO CrossReference VALUES('item5', 'crossRef2')
INSERT INTO CrossReference VALUES('item5', 'crossRef3')
/* End of test data setup */
/* Begin of actual query */
DECLARE @xRefs VARCHAR(2000),
@query VARCHAR(8000)
SELECT @xRefs = STUFF((SELECT DISTINCT '],[' + ltrim(CrossReferenceNumber)
FROM CrossReference
ORDER BY '],[' + ltrim(CrossReferenceNumber)
FOR XML PATH('')
), 1, 2, '') + ']'
SET @query =
'SELECT *
FROM Item i
INNER JOIN
(
SELECT *
FROM
(
SELECT Item, CrossReferenceNumber
FROM CrossReference
) t
PIVOT (MAX(CrossReferenceNumber) FOR CrossReferenceNumber IN (' + @xRefs + ')) as pvt
) xRefs
ON i.Item = xRefs.Item
ORDER BY i.Item'
EXECUTE (@query)
/* end */
