在查询值上联接两个SQL表的最聪明方法

Question

I have two tables in Sql Server 2008

Items
------
ItemNo
StartDate


Translation
-------
ItemNo
StartDate
Language
Description

In the Translations table there are multiple translations for each product, ie.

ItemNo: 123
StartDate: 2011-11-02
Language: SE
Description: 'Bla1'
ItemNo: 123
StartDate: 2011-11-02
Language: DK
Description: 'Bla2'

How can I build a fast select statement that outputs a single line for each product without using too many lookups, like

    Itemno  StartDate  DK     SE
------------------------------------
    123     2011-11-02 Bla1   Bla2

Thanx in advance Jens

Answer 1

Here is a version using XML to do what you want. I have not tested the performance against the other methods suggested in the comment sections.

Can't see why StartDate should be in both tables other than that it is part of the PK in Items table. If StartDate is not part of the PK you can safely remove it from all joins.

declare @Items table
(
  ItemNo int,
  StartDate date
)

declare @Translation table
(
  ItemNo int,
  StartDate date,
  Language varchar(2),
  Description varchar(20)
)

insert into @Items values
(123, '2011-11-02')

insert into @Translation values
(123, '2011-11-02', 'DK', 'Bla1'),
(123, '2011-11-02', 'SE', 'Bla2')


select I.ItemNo,
       I.StartDate,
       T.Trans.value('(Trans[@Language="DK"]/Desc)[1]', 'varchar(20)') as DK,
       T.Trans.value('(Trans[@Language="SE"]/Desc)[1]', 'varchar(20)') as SE
from @Items as I
  inner join (select T1.ItemNo,
                     T1.StartDate,
                     (select T2.Language as '@Language',
                             T2.Description as 'Desc'
                      from @Translation as T2
                      where T1.ItemNo = T2.ItemNo and
                            T1.StartDate = T2.StartDate
                      for xml path('Trans'), type) as Trans
              from @Translation as T1
              group by T1.ItemNo,
                       T1.StartDate) as T
    on I.ItemNo = T.ItemNo and
       I.StartDate = T.StartDate