http://www.codeproject.com/Articles/818694/SQL-queries-to-manage-hierarchical-or-parent-child
Introduction
Here in this post we will try to manage data’s with hierarchical relation or parent-child relation of a specific table in SQL server. Our special concentration would be over,
- Show Generations of each row
- Find all possible parents of a specific row
- Find all possible child’s of a specific row
- Show all possible parents at a column with a separator
- Show all possible child’s at a column with a separator
Background
Let’s pretend
- A row can have only one parent or Null as no parent
- There is at least a row, without parent (parentId is null)
- And at least a row, without any child
Here is our table schema.
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CREATE TABLE UserType(
Id BIGINT NOT NULL,
Name VARCHAR(100) NOT NULL,
ParentId BIGINT NULL
)
Let’s populate the table with data’s
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DECLARE @maxCount BIGINT,
@count BIGINT,
@parentId BIGINT;
SET @maxCount = 10;
SET @count = 1;
WHILE @count <= @maxCount
BEGIN
If @count = 1
SET @parentId = NULL;
ELSE
SET @parentId = @count - 1;
INSERT INTO UserType(Id, Name, ParentId)
VALUES (@count, 'User_' + CONVERT(VARCHAR(400), @count), @parentId)
SET @count = @count + 1;
END
So our populated table would be like
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SELECT *
FROM UserType;
Now how to find all this generations, parents or child’s using SQL for a specific row …!!!
The answer is using recursion. But to use this recursion we need some Thing Called CTE (Common Table Expressions) or in syntax “WITH” in SQL. If we don’t have any idea about it, we can start with the links or Google for few moments.
So let’s start with pretty basics.
Regular Join
Joining table with itself based on condition, where ones parentId is equal to another’s Id
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SELECT ChildUserType.Id, ChildUserType.Name, ParentUserType.Id, ParentUserType.Name
FROM UserType AS ChildUserType
LEFT JOIN UserType AS ParentUserType ON ChildUserType.ParentId = ParentUserType.Id;
Row Generation
The procedure is something like
- All rows with no parent (NULL), assign generation 0 to them
- Find rows where parent belongs to the generation 0, and assign increased generation to itself
- Do until the recursion is finished.
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WITH Hierarchy(ChildId, ChildName, Generation, ParentId)
AS
(
SELECT Id, Name, 0, ParentId
FROM UserType AS FirtGeneration
WHERE ParentId IS NULL
UNION ALL
SELECT NextGeneration.Id, NextGeneration.Name, Parent.Generation + 1, Parent.ChildId
FROM UserType AS NextGeneration
INNER JOIN Hierarchy AS Parent ON NextGeneration.ParentId = Parent.ChildId
)
SELECT *
FROM Hierarchy
OPTION(MAXRECURSION 32767)
All Possible Parents
Here we are trying to find all possible parents of a row where it's Id = 5
- Starts with selecting the row where Id = 5
- Find other rows where its id is equal to previously selected’s ParentId
- And continue reduction
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DECLARE @id BIGINT;
SET @id = 5;
WITH tblParent AS
(
SELECT *
FROM UserType WHERE Id = @id
UNION ALL
SELECT UserType.*
FROM UserType JOIN tblParent ON UserType.Id = tblParent.ParentId
)
SELECT * FROM tblParent
WHERE Id <> @id
OPTION(MAXRECURSION 32767)
All Possible Childs
Here we are trying to find all possible child’s of a row where it’s Id = 5
- Starts with selecting the row where Id = 5
- Find other rows where its ParentId is equal to previously selected’s Id
- And continue reduction
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DECLARE @userTypeId BIGINT;
SET @userTypeId = 5;
WITH tblChild AS
(
SELECT *
FROM UserType WHERE ParentId = @userTypeId
UNION ALL
SELECT UserType.* FROM UserType JOIN tblChild ON UserType.ParentId = tblChild.Id
)
SELECT *
FROM tblChild
OPTION(MAXRECURSION 32767)
All Possible Parents In A Column
Here we are showing all the possible parents Ids at the column with a specific separator ‘.’
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WITH Hierarchy(ChildId, ChildName, ParentId, Parents)
AS
(
SELECT Id, Name, ParentId, CAST('' AS VARCHAR(MAX))
FROM UserType AS FirtGeneration
WHERE ParentId IS NULL
UNION ALL
SELECT NextGeneration.Id, NextGeneration.Name, Parent.ChildId,
CAST(CASE WHEN Parent.Parents = ''
THEN(CAST(NextGeneration.ParentId AS VARCHAR(MAX)))
ELSE(Parent.Parents + '.' + CAST(NextGeneration.ParentId AS VARCHAR(MAX)))
END AS VARCHAR(MAX))
FROM UserType AS NextGeneration
INNER JOIN Hierarchy AS Parent ON NextGeneration.ParentId = Parent.ChildId
)
SELECT *
FROM Hierarchy
OPTION(MAXRECURSION 32767)
All Possible Childs In A Column
Here we are showing all the possible child’s Ids at the column with a specific separator ‘.’
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WITH Hierarchy(ChildId, ChildName, ParentId, Childs)
AS
(
SELECT Id, Name, ParentId, CAST('' AS VARCHAR(MAX))
FROM UserType AS LastGeneration
WHERE Id NOT IN (SELECT COALESCE(ParentId, 0) FROM UserType)
UNION ALL
SELECT PrevGeneration.Id, PrevGeneration.Name, PrevGeneration.ParentId,
CAST(CASE WHEN Child.Childs = ''
THEN(CAST(Child.ChildId AS VARCHAR(MAX)))
ELSE(Child.Childs + '.' + CAST(Child.ChildId AS VARCHAR(MAX)))
END AS VARCHAR(MAX))
FROM UserType AS PrevGeneration
INNER JOIN Hierarchy AS Child ON PrevGeneration.Id = Child.ParentId
)
SELECT *
FROM Hierarchy
OPTION(MAXRECURSION 32767)
Recursion limit !!!
In all of the previous queries we have used syntax like
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OPTION(MAXRECURSION 32767)
This specifies the maximum number of recursions in CTE. Now if we don’t use this OPTION(MAXRECURSION 32767) , by default it is 100.
We need to specify this number depending on the recursion requirement.
