SQL Queries to Manage Hierarchical or Parent-child Relational Rows in SQL Server

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Introduction

Here, in this post, we will try to manage data 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 childs 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:

/*drop the tbl*/
--DROP TABLE UserType

/*create the tbl*/
CREATE TABLE UserType(
    Id BIGINT NOT NULL,
    Name VARCHAR(100) NOT NULL,
    ParentId BIGINT NULL    
)

Let’s populate the table with data:

/*insert data into table*/
DECLARE @maxCount BIGINT,
        @count BIGINT,
        @parentId BIGINT;        
SET @maxCount = 10;        /*change to input more*/
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:

/*show inserted datas*/
SELECT *
    FROM UserType;

Check in live http://www.sqlfiddle.com/#!3/f50a6/1

Now how to find all these generations, parents or childs using SQL for a specific row …!!!

The answer is using recursion. But to use this recursion, we need something 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.

• http://msdn.microsoft.com/en-us/library/ms175972.aspx
• http://technet.microsoft.com/en-us/library/ms190766%28v=sql.105%29.aspx
• http://www.codeproject.com/Articles/265371/Common-Table-Expressions-CTE-in-SQL-SERVER

So let’s start with pretty basic stuff.

Regular Join

Joining table with itself based on condition, where ones parentId is equal to another’s Id.

/*regular join to get detail*/    
SELECT ChildUserType.Id, ChildUserType.Name, ParentUserType.Id, ParentUserType.Name
    FROM UserType AS ChildUserType
    LEFT JOIN UserType AS ParentUserType ON ChildUserType.ParentId = ParentUserType.Id;

Check in live http://www.sqlfiddle.com/#!3/f50a6/2

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

/*row generations*/
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)

Check in live http://www.sqlfiddle.com/#!3/f50a6/3

All Possible Parents

Check http://stackoverflow.com/a/21233413/2948523

Here, we are trying to find all possible parents of a row where its Id = 5.

• Starts with selecting the row where Id = 5
• Find other rows where its id is equal to previously selected ParentId
• And continue reduction

--all possible parents of @id
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)

Check in live http://www.sqlfiddle.com/#!3/f50a6/5

All Possible childs

Check http://stackoverflow.com/a/21233413/2948523

Here, we are trying to find all possible childs of a row where its Id = 5.

• Starts with selecting the row where Id = 5
• Find other rows where its ParentId is equal to previously selected Id
• And continue reduction

-- all possible childs of @userTypeId
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)

Check in live http://www.sqlfiddle.com/#!3/f50a6/6

All Possible Parents in a Column

Here, we are showing all the possible parent Ids at the column with a specific separator ‘.’

/*row possible parents in a column*/
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)

Check in live http://www.sqlfiddle.com/#!3/f50a6/7

All Possible Childs in a Column

Here, we are showing all the possible child's Ids at the column with a specific separator ‘.’

/*row possible childs in a column*/
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)

Check in live http://www.sqlfiddle.com/#!3/f50a6/8

Recursion Limit !!!

In all of the previous queries, we have used syntax like:

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.

If More Recursion Needed !!!

With MAXRECURSION value equal to 0 means that no limit is applied to the recursion level, but remember a recursion should end at some level.

OPTION(MAXRECURSION 0)

To increase the recursion number, excluding CTE’s maximum Limit. We can follow some instruction like the links, or Google for some time.

• http://www.sql-server-helper.com/error-messages/msg-310.aspx
• http://stackoverflow.com/questions/15080922/infinite-loop-cte-with-option-maxrecursion-0

Limitations

Yes, there could be something which I misunderstood or presented. So if you find anything, just let me know.

Find the SQL file as attachment.