One of the most benefit of CTE (Common Table Expressions) is that we can create
recursive queries with them.
Recursive query is the efficient way to display hierarchy setup (Parent child relation
ship in the same table). e.g. Grand Parent, Parent, Child. This is common requirement.
To display the parent child relationship in efficient manner, we can use Recursive
CTE. Let me explain it by example.
Create Temporary Table:
DECLARE @Images TABLE ( [ResourceID] [int] NOT NULL, [ChildOf] [int] NOT NULL, [ResourceName] [varchar](250) NOT NULL )
Lets populate some data into this table:
INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (5, 0, N'Mammal') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (6, 5, N'Carnivore') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (14, 6, N'Cannine') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (15, 14, N'Dog') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (16, 15, N'Dog1') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (17, 14, N'Monkey') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (18, 17, N'Monkey1') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (19, 16, N'dog1.01') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (21, 18, N'monkey1.02') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (23, 17, N'test') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (25, 17, N'Monkey 2') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (31, 0, N'Fish') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (32, 31, N'cartilaginous fish') INSERT @Images ([ResourceID], [ChildOf], [ResourceName]) VALUES (33, 32, N'dogfish')
That is how data is stored in the table:
This is not the way that we can understand the hierarchy. Recursive query will help
us to display result like this:
This is much efficient way to list out, as we understand that how the hierarchy
is being maintained. Recursive CTE helps us to generate this kind efficient output.
Let me share the t-sql script which generates the output which we are looking for:
;WITH images
AS (
SELECT *
,ROW_NUMBER() OVER (
ORDER BY ResourceID
) AS RowID
FROM @images
)
,cte
AS (
SELECT 1 AS [Level]
,[ResourceID]
,[ResourceName]
,ChildOf
,cast(cast(RowID AS VARBINARY(4)) AS VARBINARY(max)) AS sort
FROM images
WHERE ChildOf = 0
--where [ResourceID] = 5
UNION ALL
SELECT p.[Level] + 1
,c.[ResourceID]
,c.[ResourceName]
,c.ChildOf
,cast(p.sort + cast(c.RowID AS VARBINARY(4)) AS VARBINARY(max))
FROM images c
INNER JOIN cte p ON p.[ResourceID] = c.ChildOf
--WHERE c.ChildOF=5
)
SELECT ResourceID
,ChildOf
,REPLICATE('.', ([Level] - 1) * 4) + [ResourceName] AS menu
FROM cte
ORDER BY sort
That's it. Here, I have used first CTE, to assign the Unique numbers. Then just
iterate thru each record and find their related children records. Hope this helps
you.
Reference : Tejas Shah http://www.SQLYoga.com
