Why is my left join not returning nulls?

Question

In sql server 2008, I have the following query:

select      
    c.title as categorytitle,
    s.title as subcategorytitle,
    i.title as itemtitle
from cat         


        

Answer 1

change join items i... to LEFT join items i... and your query should work as you expect.

EDIT
You can not filter LEFT JOIN tables in the where clause unless you account for nulls, because the left join allows those columns to have a value or be null when no rows matches:

and i.siteid = 132 will throw away any of your rows that have a NULL i.siteid, where none existed. Move this to the ON:

left join items i on ic.itemid = i.itemid and i.siteid = 132

or make the WHERE handle NULLs:

WHERE ... AND (i.siteid = 132 OR i.siteid IS NULL)

EDIT based on OP's edit 3

SET NOCOUNT ON
DECLARE @Categories table (CategoryID int,Title varchar(30))
INSERT @Categories VALUES (1,'Cat AAA')
INSERT @Categories VALUES (2,'Cat BBB')
INSERT @Categories VALUES (3,'Cat CCC')

DECLARE @SubCategories table (SubCategoryID int,CategoryID int,Title varchar(30))
INSERT @SubCategories VALUES (1,1,'SubCat AAA A')
INSERT @SubCategories VALUES (2,1,'SubCat AAA B')
INSERT @SubCategories VALUES (3,1,'SubCat AAA C')
INSERT @SubCategories VALUES (4,2,'SubCat BBB A')

DECLARE @ItemCategories table (ItemCategoryID int, ItemID int, SubCategoryID int, IsActive char(1))
INSERT @ItemCategories VALUES (1,1,2,'Y')
INSERT @ItemCategories VALUES (2,2,2,'Y')
INSERT @ItemCategories VALUES (3,3,2,'Y')
INSERT @ItemCategories VALUES (4,4,2,'Y')
INSERT @ItemCategories VALUES (5,7,2,'Y')

DECLARE @Items table (ItemID int, Title varchar(30), SiteID int)
INSERT @Items VALUES (1,'Item A',111)
INSERT @Items VALUES (2,'Item B',111)
INSERT @Items VALUES (3,'Item C',132)
INSERT @Items VALUES (4,'Item D',111)
INSERT @Items VALUES (5,'Item E',111)
INSERT @Items VALUES (6,'Item F',132)
INSERT @Items VALUES (7,'Item G',132)
SET NOCOUNT OFF

I'm not 100% sure what the OP is after, this will return all info that can be joined when the siteid=132 as given in the question

SELECT
    c.title as categorytitle
        ,s.title as subcategorytitle
        ,i.title as itemtitle
        --,i.itemID, ic.SubCategoryID, s.CategoryID
    FROM @Items                          i
        LEFT OUTER JOIN @ItemCategories ic ON i.ItemID=ic.ItemID
        LEFT OUTER JOIN @SubCategories   s ON ic.SubCategoryID=s.SubCategoryID
        LEFT OUTER JOIN @Categories      c ON s.CategoryID=c.CategoryID
    WHERE i.siteid = 132

OUTPUT:

categorytitle                  subcategorytitle               itemtitle
------------------------------ ------------------------------ ------------------------------
Cat AAA                        SubCat AAA B                   Item C
NULL                           NULL                           Item F
Cat AAA                        SubCat AAA B                   Item G

(3 row(s) affected)

This will list all categories, even if there is no match to the siteid=132

;WITH AllItems AS
(
SELECT
    s.CategoryID, ic.SubCategoryID, ItemCategoryID, i.ItemID
        ,c.title AS categorytitle, s.title as subcategorytitle, i.title as itemtitle
    FROM @Items                          i
        LEFT OUTER JOIN @ItemCategories ic ON i.ItemID=ic.ItemID
        LEFT OUTER JOIN @SubCategories   s ON ic.SubCategoryID=s.SubCategoryID
        LEFT OUTER JOIN @Categories      c ON s.CategoryID=c.CategoryID
    WHERE i.siteid = 132
)
SELECT
    categorytitle, subcategorytitle,itemtitle
    FROM AllItems
UNION
SELECT
    c.Title, s.Title, null
    FROM @Categories                     c
        LEFT OUTER JOIN @SubCategories   s ON c.CategoryID=s.CategoryID
        LEFT OUTER JOIN @ItemCategories ic ON s.SubCategoryID=ic.SubCategoryID
        LEFT OUTER JOIN AllItems         i ON c.CategoryID=i.CategoryID AND  s.SubCategoryID=i.SubCategoryID
    WHERE i.ItemID IS NULL
ORDER BY categorytitle,subcategorytitle

OUTPUT:

categorytitle                  subcategorytitle               itemtitle
------------------------------ ------------------------------ ------------------------------
NULL                           NULL                           Item F
Cat AAA                        SubCat AAA A                   NULL
Cat AAA                        SubCat AAA B                   Item C
Cat AAA                        SubCat AAA B                   Item G
Cat AAA                        SubCat AAA C                   NULL
Cat BBB                        SubCat BBB A                   NULL
Cat CCC                        NULL                           NULL

(7 row(s) affected)