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SQL Server: Use of Recursive Query with example
12 Aug 2015
Tejas Shah
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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.

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Tejas Shah

Microsoft Certified Professional Expert. I have 14+ years of experience in Design and Develop 100+ .Net applications using Asp.Net, C#, Asp.Net Core, VB.Net, SQL Server, MVC, AngularJS, JavaScript, Azure, HTML5 and CSS3.

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