为什么我的左联接不返回空值? [英] Why is my left join not returning nulls?
问题描述
在sql server 2008中，我有以下查询:
select
c.title as categorytitle,
s.title as subcategorytitle,
i.title as itemtitle
from categories c
join subcategories s on c.categoryid = s.categoryid
left join itemcategories ic on s.subcategoryid = ic.subcategoryid
left join items i on ic.itemid = i.itemid and i.siteid = 132
where (ic.isactive = 1 or ic.isactive is null)
order by c.title, s.title
我正在尝试获取其子类别中的项目，但是如果该类别或子类别中没有项目，我仍然想返回一条记录.没有项目的子类别永远不会返回.我在做什么错了?
谢谢
编辑
修改后的查询带有第二个左连接和where子句，但它仍未返回null. :/
编辑2
将站点ID移至左连接项.当我这样做时，我会得到比预期更多的记录.有些项目的网站ID为空，我只想在它们具有特定ID时才包含它们.
编辑3
表结构:
Categories Table
-------
CategoryID
Title
SubCategories Table
-------
SubCategoryID
CategoryID
Title
ItemCategories Table
-------
ItemCategoryID
ItemID
SubCategoryID
IsActive
Items Table
--------
ItemID
Title
SiteID
将join items i ...更改为LEFT join items i ...，您的查询应该可以按预期工作.
编辑
除非您考虑空值，否则您不能在where子句中过滤LEFT JOIN表，因为左连接允许这些列具有值，或者当没有行匹配时为空:
and i.siteid = 132将丢弃所有没有NULL i.siteid的行.将其移至开":
left join items i on ic.itemid = i.itemid and i.siteid = 132
或使WHERE处理NULL:
WHERE ... AND (i.siteid = 132 OR i.siteid IS NULL)
EDIT (基于OP的修改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
我不确定100％是OP，这将返回问题中给出的siteid=132时可以加入的所有信息
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
输出:
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)
这将列出所有类别，即使与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
输出:
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)
In sql server 2008, I have the following query:
select
c.title as categorytitle,
s.title as subcategorytitle,
i.title as itemtitle
from categories c
join subcategories s on c.categoryid = s.categoryid
left join itemcategories ic on s.subcategoryid = ic.subcategoryid
left join items i on ic.itemid = i.itemid and i.siteid = 132
where (ic.isactive = 1 or ic.isactive is null)
order by c.title, s.title
I am trying to get items in their subcategories, but I still want to return a record if there are no items in the category or subcategory. Subcategories that have no items are never returned. What am I doing wrong?
Thank you
EDIT
Modified query with a second left join and where clause, but it's still not returning nulls. :/
EDIT 2
Moved siteid to item left join. When I do this I get way more records than expected. Some items have a null siteid and I only want to included them when they have a specific id.
EDIT 3
Table structure:
Categories Table
-------
CategoryID
Title
SubCategories Table
-------
SubCategoryID
CategoryID
Title
ItemCategories Table
-------
ItemCategoryID
ItemID
SubCategoryID
IsActive
Items Table
--------
ItemID
Title
SiteID
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)
这篇关于为什么我的左联接不返回空值?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
