Conditional left joins - tsql

Question

I have two tables: ItemCategories and Items. ItemCategories list the categories for items and contain language dependent text strings. The CultureCode column determines which language the text belongs to.
ItemCategories
ID Name CultureCode
1 Food 'en-us'
2 Clothing 'en-us'
3 Computers 'en-us'
4 Books 'en-us'
1 Essen 'de-de'
2 Kleidung 'de-de'
3 Rechner 'de-de'
4 Bücher 'de-de'
Items
ID CategoryID1 CategoryID2
4 1 NULL
5 NULL 2
6 3 4
7 NULL NULL
The Items table lists each item and the values under the CategoryID1 and CategoryID2 refer to the ID field in CategoryItems. The values of CategoryID1 and CategoryID2 can be both null , both set to an ID or just one of them set to an ID value.
I need to create an SQL (T-SQL) that returns the ID from Items along with its correspond Name from ItemCategories when the CultureCode is set to a specific language. So if my CultureCode is set to 'en-us', the results should be:
ID CategoryID1 CategoryID2 CategoryName1 CategoryName2
4 1 NULL Food Null
5 NULL 2 null Clothing
6 3 4 Computers Books
7 NULL NULL null null
This is what I have so far:
Select I.*
From Items I
Left Join ItemCategories IC On (IC.ID = I.CategoryID1) Or (IC.ID = I.CategoryID2)
where (IC.CultureCode = 'en-us') Or (IC.CultureCode is null)
But this is not giving me the results I want.

Answer 1

Sounds like you need to split your LEFT JOIN into two separate LEFT JOINs:
SELECT I.ID, I.CategoryID1, I.CategoryID2, CategoryName1 = IC1.Name, CategoryName2 = IC2.Name
FROM Items I
LEFT JOIN ItemCategories IC1 ON IC1.ID = I.CategoryID1 AND IC1.CultureCode = 'en-us'
LEFT JOIN ItemCategories IC2 ON IC2.ID = I.CategoryID2 AND IC2.CultureCode = 'en-us';

Answer 2

Give a try and let me know your results/Comments.
CREATE TABLE ItemCategories
(
ID INT
,Name VARCHAR(50)
,CultureCode VARCHAR(5)
)
GO
INSERT INTO ItemCategories VALUES(1 ,'Food ' , 'en-us')
INSERT INTO ItemCategories VALUES(2 ,'Clothing' , 'en-us')
INSERT INTO ItemCategories VALUES(3 ,'Computers' , 'en-us')
INSERT INTO ItemCategories VALUES(4 ,'Books' , 'en-us')
INSERT INTO ItemCategories VALUES(1 ,'Essen' , 'de-de')
INSERT INTO ItemCategories VALUES(2 ,'Kleidung' , 'de-de')
INSERT INTO ItemCategories VALUES(3 ,'Rechner' , 'de-de' )
INSERT INTO ItemCategories VALUES(4 ,'Bücher ' , 'de-de')
CREATE TABLE Items
(
ID INT
,CategoryID1 INT
,CategoryID2 INT
)
GO
INSERT INTO Items VALUES (4 , 1 , NULL)
INSERT INTO Items VALUES (5 , NULL , 2 )
INSERT INTO Items VALUES (6 , 3 , 4 )
INSERT INTO Items VALUES (7 , NULL , NULL)
Select I.*,IC.Name,IT.Name
From Items I
left join ItemCategories IC ON (IC.ID = I.CategoryID1) AND IC.CultureCode = 'en-us'
left join ItemCategories IT On (It.ID = I.CategoryID2) AND IT.CultureCode = 'en-us'

Answer 3

You can do it in two ways:
Use subqueries:
Select
I.ID,
(SELECT TOP 1 ID FROM ItemCategories IC WHERE IC.ID=I.CategoryID1 AND IC.CultureCode = 'en-us') AS CategoryID1,
(SELECT TOP 1 ID FROM ItemCategories IC WHERE IC.ID=I.CategoryID2 AND IC.CultureCode = 'en-us') AS CategoryID2,
(SELECT TOP 1 Name FROM ItemCategories IC WHERE IC.ID=I.CategoryID1 AND IC.CultureCode = 'en-us') AS CategoryName1,
(SELECT TOP 1 Name FROM ItemCategories IC WHERE IC.ID=I.CategoryID2 AND IC.CultureCode = 'en-us') AS CategoryName2
From Items I
Or, split query into 2 joins
SELECT DISTINCT I.*, IC1.Name AS CategoryName1, IC2.Name AS CategoryName2
FROM Items I
LEFT JOIN ItemCategories IC1 ON I.CategoryID1=IC1.ID
LEFT JOIN ItemCategories IC2 ON I.CategoryID2=IC2.ID
WHERE (IC1.CultureCode IS NULL OR IC1.CultureCode = 'en-us') AND (IC2.CultureCode IS NULL OR IC2.CultureCode = 'en-us');