This blog post is inspired from SQL Queries Joes 2 Pros: SQL Query Techniques For Microsoft SQL Server 2008 – SQL Exam Prep Series 70-433 – Volume 2.
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What is a Common Table Expression (CTE)
A CTE can be thought of as a temporary result set and are similar to a derived table in that it is not stored as an object and lasts only for the duration of the query. A CTE is generally considered to be more readable than a derived table and does not require the extra effort of declaring a Temp Table while providing the same benefits to the user. However; a CTE is more powerful than a derived table as it can also be self-referencing, or even referenced multiple times in the same query.
The basic syntax structure for a CTE is shown below:
WITH MyCTE
AS ( SELECT EmpID, FirstName, LastName, ManagerID
FROM Employee
WHERE ManagerID IS NULL )
SELECT *
FROM MyCTE
Building a Recursive CTE
In the following examples, you will learn how to harness the power of a recursive CTE query by fulfilling a common business requirement, retrieving hierarchical data. By the time the final query is complete you will be able to easily determine how many levels from the top executive each employee is.
A recursive CTE requires four elements in order to work properly.
- Anchor query (runs once and the results ‘seed’ the Recursive query)
- Recursive query (runs multiple times and is the criteria for the remaining results)
- UNION ALL statement to bind the Anchor and Recursive queries together.
- INNER JOIN statement to bind the Recursive query to the results of the CTE.
WITH MyCTE
AS ( SELECT EmpID, FirstName, LastName, ManagerID
FROM Employee
WHERE ManagerID IS NULL
UNION ALL
SELECT EmpID, FirstName, LastName, ManagerID
FROM Employee
INNER JOIN MyCTE ON Employee.ManagerID = MyCTE.EmpID
WHERE Employee.ManagerID IS NOT NULL )
SELECT *
FROM MyCTE
Identify the Anchor and Recursive Query
Anyone who does not have a boss is considered to be at the top level of the company and everyone who does have a boss either works for the person(s) at the top level (upper management), or the people that work for them (mid-management thru base employees).
For example, a CEO is at the top level and thus has a ManagerID of null. Likewise, everyone below the CEO will have a ManagerID. This is demonstrated in the two queries below:
The first SELECT statement will become your Anchor query as it will find the employee that has a ManagerID of null (representing Level 1 of the organization). The second SELECT statement will become your Recursive query and it will find all employees that do have a ManagerID (representing Level 2-3 of this organization).
As you can see from the results so far, these queries are unable to give hierarchical data on which level each employee is at within the organization.
Add the Anchor and Recursive query to a CTE
Begin transforming this entire query into a CTE by placing a UNION ALL statement between the Anchor and Recursive queries. Now add parentheses around the entire query, indenting it, moving it down, and adding the declaration WITH EmployeeList AS before the open parenthesis, and then add SELECT * FROM EmployeeList on the next line after the close parenthesis.
Your query should now look like the screenshot below:
As you can see, the results from your CTE are exactly the same as the results returned from running the anchor and Recursive queries simultaneously in the previous example.
Add an expression to track hierarchical level
The Anchor query (aliased as ‘Boss’) inside the CTE represents everyone at Level 1 (i.e. Sally Smith). The Recursive query (aliased as ‘Emp’) represents everyone at Levels 2 and 3. In order to visualize each level in a result set, you will need to add an expression field to each query.
Add the expression “1 AS EmpLevel” to the Anchor query and the expression “2 AS EmpLevel” to the Recursive query. Before executing the entire query, look closely at the expression field. The EmpLevel expressions in the Anchor query will hard-code the numeral 1 (for Sally Smith’s level), while the EmpLevel expressions in the Recursive query will hard-code the numeral 2 for everyone else.
Your query should now look like the screenshot below:
The two new expression fields were a helpful step. In fact, they show the correct EmpLevel information for Sally Smith and for the people at Level 2 (i.e., Adams, Bender, Brown, Kennson, Lonning and Osako). However, the 2 is just a hard-coded placeholder to help visualize your next step. Lisa Kendall and several other employees need to be at Level 3.
Ideally you would like to make the expression dynamic by replacing “2 AS EmpLevel” with the expression “EmpLevel + 1”.
Add a self-referencing INNER JOIN statement
Let’s take a moment and recognise why this is not going to work quite so simply. The idea to increment EmpLevel in the recursive query of the CTE is on the right track. Unfortunately, the recursive query is trying to reference a field called EmpLevel but can’t find one, since it has only been materialized in the result set of the Anchor query and does not yet exist in the recursive set.
How can you materialize the EmpLevel field for the recursive query? We can use the CTE for this! Remember, a recursive CTE requires an INNER JOIN to connect the recursive query to the CTE itself. Go ahead and write an INNER JOIN statement binding the recursive query ‘Emp’ to the CTE ‘EmployeeList AS EL’ ON Emp.ManagerID = EL.EmpID.
Your query should now look like the screenshot below:
Success! You can now see that Sally is at the first level, Alex is at the second level and Lisa appears at the third level. Since a CTE can reference itself, the ‘Emp’ recursive query can now access the EmpLevel field materialized in the EmployeeList CTE.
This blog post is inspired from SQL Queries Joes 2 Pros: SQL Query Techniques For Microsoft SQL Server 2008 – SQL Exam Prep Series 70-433 – Volume 2.
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Reference: Pinal Dave (https://blog.sqlauthority.com)
74 Comments. Leave new
An expert solution. Thank you!
Dear Pinal,
I am facing a performance challenge using CTE.
Let me explain the scenario.
I am calling in a SP using .Net program’s in line query.
in this SP first we are extracting data from XML which are of size minimum 500 KB to 4 MB each approximately 30,000 files. later we are using concept of CTE to make records unique.
but here performance issue comes. some times it takes 30-40 minutes to execute the main SP. I just want to know where i made mistake. Ur help will be really appreciated.
Here i am sending 2 SPs (Main and CTE one)
CREATE PROCEDURE [dbo].[usp_Read_BSXML] (
@XmlDocumentBS xml,
@CIN varchar(200),
@FindateID int,
@prm_error_message VARCHAR(250) = ” OUTPUT
)
AS
DECLARE @XmlDocumentHandle int
BEGIN
BEGIN TRANSACTION
BEGIN TRY
/*– Create an internal representation of the XML document.*/
EXEC sp_xml_preparedocument @XmlDocumentHandle OUTPUT, @XmlDocumentBS, ”
— Insert record into XBRL_BS_TMP table
INSERT INTO XBRL_BS_TMP (CIN,contextRef, NodeName, NodeValue, NodeType, FinDate,FindateId)
SELECT @CIN as CIN,
Cast(c.n.value(‘@contextRef’ ,’varchar(1000)’) as varchar(1000)) as contextRef,
Cast(n.value(‘local-name(.)’,’varchar(4000)’) as varchar(4000)) as NodeName,
Cast(c.n.value(‘.’,’varchar(MAX)’) as varchar(max)) as NodeValue ,
cast(n.query(‘namespace-uri(.)’) as varchar(200)) as NodeType,
substring(cast(n.query(‘namespace-uri(.)’) as varchar(100)),35,10) as FinDate,
@FindateID as FindateId
from @XmlDocumentBS.nodes(‘//*’) as C(N)
Where c.n.value(‘@contextRef’ ,’varchar(100)’) is not null;
— Insert record into XBRL_BS_FINDATE table
INSERT INTO XBRL_BS_FINDATE (FindateId,CIN,contextRefId,StartDate,EndDate,InstantDate)
SELECT @FindateID as FindateId,@CIN as CIN,
Cast(t.n.value(‘@id’ ,’varchar(1000)’) as varchar(1000)) as contextRefId,
Cast(n.value(‘(*:period/*:startDate/text())[1]’,’varchar(1000)’) as varchar(1000)) as StartDate,
Cast(n.value(‘(*:period/*:endDate/text())[1]’,’varchar(1000)’) as varchar(1000)) as EndDate,
Cast(n.value(‘(*:period/*:instant/text())[1]’,’varchar(1000)’) as varchar(1000)) as InstantDate
from @XmlDocumentBS.nodes(‘//*:context’) as T(N)
Where t.n.value(‘@contextRef’,’varchar(MAX)’) is null and t.n.value(‘@id’ ,’varchar(MAX)’) is not null;
— To insert explicitMember node value and dimension attribute value
Insert into XBRL_BS_SCENARIO_DATA(FindateId,CIN,contextRefId,dimension,explicitMember,typedMember)
SELECT @FindateID as FindateId,@CIN as CIN,*
from (SELECT
context.Node.value(‘@id’, ‘varchar(1000)’) ‘contextRefId’
,items.Cd.value(‘@dimension’, ‘varchar(1000)’) ‘dimension’
,items.Cd.value(‘.’, ‘VARCHAR(1000)’) ‘explicitMember’
,items.Cd.value(‘.’, ‘VARCHAR(1000)’) ‘typedMember’
FROM
@XmlDocumentBS.nodes(‘//*:context’) context(Node)
CROSS APPLY context.Node.nodes(‘./*:scenario/*:explicitMember’) items(Cd)
Where context.Node.value(‘@contextRef’,’varchar(MAX)’) is null and context.Node.value(‘@id’ ,’varchar(MAX)’) is not null
UNION ALL
SELECT
context.Node.value(‘@id’, ‘varchar(1000)’) ‘contextRefId’
,items.Cd.value(‘@dimension’, ‘varchar(1000)’) ‘dimension’
,items.Cd.value(‘.’, ‘VARCHAR(1000)’) ‘explicitMember’
,items.Cd.value(‘.’, ‘VARCHAR(1000)’) ‘typedMember’
FROM
@XmlDocumentBS.nodes(‘//*:context’) context(Node)
CROSS APPLY context.Node.nodes(‘./*:scenario/*:typedMember’) items(Cd)
Where context.Node.value(‘@contextRef’,’varchar(MAX)’) is null and context.Node.value(‘@id’ ,’varchar(MAX)’) is not null
) AS tbl
EXEC usp_INSERT_ON_O_BS_DEFINITION @CIN,@FindateID;
EXEC sp_xml_removedocument @XmlDocumentHandle OUTPUT;
COMMIT
END TRY
BEGIN CATCH
SET @prm_error_message = ERROR_MESSAGE()
select @prm_error_message,0
ROLLBACK
END CATCH
END
GO
=====================================================================
usp_INSERT_ON_O_BS_DEFINITION (To make my info unique)
USE [XBRLNew_02]
GO
/****** Object: StoredProcedure [dbo].[usp_INSERT_ON_O_BS_DEFINITION] Script Date: 10/06/2015 15:29:20 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE [dbo].[usp_INSERT_ON_O_BS_DEFINITION] (
@CIN VARCHAR(200),
@finDateId int,
@prm_error_message VARCHAR(250) = ” OUTPUT
)
AS
BEGIN
BEGIN TRANSACTION
BEGIN TRY
;WITH BS_CTE (RowNumber, CIN, contextRefId, FindateID, explicitMember, explicitMembers, dimesnsion, dimensions) AS
(
SELECT 1, BS_SC.CIN, BS_SC.contextRefId, BS_SC.FindateId, MIN(BS_SC.explicitMember),
CAST(MIN(BS_SC.explicitMember) AS VARCHAR(8000)),
MIN(BS_SC.dimension),
CAST(MIN(BS_SC.dimension) AS VARCHAR(8000))
FROM XBRL_BS_SCENARIO_DATA BS_SC
WHERE BS_SC.CIN =@CIN
and BS_SC.findateId = @findateId
GROUP BY BS_SC.FindateId,BS_SC.CIN,BS_SC.contextRefId
UNION ALL
SELECT C.RowNumber + 1, B.CIN, B.contextRefId, B.FindateId, B.explicitMember, C.explicitMembers+’, ‘ + B.explicitMember,
B.dimension, C.dimensions+’, ‘ + B.dimension
FROM XBRL_BS_SCENARIO_DATA B, BS_CTE C
WHERE B.CIN = C.CIN
And B.FindateId = @findateId
AND B.FindateId = C.FindateID AND B.contextRefId = C.contextRefId
AND B.CIN = @CIN AND B.explicitMember > C.explicitMember)
INSERT INTO O_XBRL_BS_DFINITION (CIN, FindateId, contextRefId, FYDate,explicitMembers,dimensions,RowNumber)
(
SELECT FY_Date.CIN, FY_Date.FindateId, FY_Date.contextRefId ,
CASE WHEN FY_Date.StartDate IS NULL THEN FY_Date.InstantDate
ELSE FY_Date.EndDate END AS FYDate,
Scenario.explicitMembers, Scenario.dimensions, Scenario.RowNumber
FROM
XBRL_BS_FINDATE FY_Date LEFT OUTER JOIN
(SELECT BS_CTE.RowNumber, BS_CTE.CIN,BS_CTE.FindateID, BS_CTE.contextRefId, BS_CTE.explicitMembers, BS_CTE.dimensions
FROM BS_CTE ,
(SELECT Z.CIN, Z.FindateID, Z.contextRefId, MAX(Z.RowNumber) As MaxRow
FROM BS_CTE Z GROUP BY Z.FindateID,Z.CIN, Z.contextRefId ) AS SummaryRow
WHERE SummaryRow.FindateID = BS_CTE.FindateID AND SummaryRow.CIN = BS_CTE.CIN AND SummaryRow.contextRefId = BS_CTE.contextRefId
AND SummaryRow.MaxRow = BS_CTE.RowNumber) Scenario ON
FY_Date.CIN = Scenario.CIN AND FY_Date.FindateId = Scenario.FindateID AND FY_Date.contextRefId = Scenario.contextRefId
WHERE FY_Date.CIN = @CIN
And FY_Date.FindateId = @findateId
)
COMMIT
END TRY
BEGIN CATCH
SET @prm_error_message = ERROR_MESSAGE()
select @prm_error_message,0
ROLLBACK
END CATCH
END
GO
Great Article Pinal. I’m trying to get this to work where the Top Level Parents have various versions and I need to select the first one (which I can using Partition). I then need the children of just those top level parents. I correctly get those, but the second half of the Union brings in the children of all of them. Thanks. Chris.
Hi, i have a data like this
id, parent id, child id
1,1,2
2,1,3
3,2,4
4,2,5
5,3,6
6,3,7
7,8,9
8,8,10
9,9,11
10,9,12
11,10,13
12,10,14
and I need via a query results like
1|2
1|2|4
1|2|5
1|3
1|3|6
1|3|7
8|9
8|9|11
8|9|12
8|10
8|10|13
8|10|14
Did you manage to get a solution?
Thanks you Pinal, this was very helpful.
Great article Pinal. I have a question. I want to add a new column called ParentId and update it with top level parent I’d (in the above example, all records except EmpId 11 would have ParentId as 11. Can you please help me to do it. (In my table there are millions of records. )
How to achieve below with Common Table Expression (CTE).
Problem –
ParentCustomerNo ChildCustomerNo
123456 343432
343432 888888
888888 999999
999999 666666
777777 111111
111111 222222
787878 898989
898989 656565
656565 454545
Result –
Child Top Parent
222222 777777
666666 123456
999999 123456
888888 123456
454545 787878
656565 787878
898989 787878
Please let me know.
Is it better to use CTE with hierarchical id on the table ?
Could you build this example with hierarchyid ?
How to achieve below with CTE or other solution.
Table with parent and child…. Wanted to retrive child and top parent.
Table –
ParentCustomerNo ChildCustomerNo
123456 343432
343432 888888
888888 999999
999999 666666
777777 111111
111111 222222
787878 898989
898989 656565
656565 454545
Result –
Child Top_Parent
222222 777777
666666 123456
999999 123456
888888 123456
454545 787878
656565 787878
898989 787878
Thanks for the example. Should it work equally well when multiple records qualify as anchors – e.g. there is no CEO as such but, for example, half a dozen board members at level 0 where each board member oversees a tree structure of employees – senior managers, manager, base employee etc?
Hi,
I am facing the same scenario.
Please let me know if you have found the resolution on the same.
you are a very best!!!
How can we show the data in a particular order
Kelly Smith
David Kenneson
Lisa Kenson
John
Barabara
what’s logic?
Hi Pinal Thank you for this article. I am looking for the reverse..that is find all recursive children of a parent in the hierarchical data. So in your employee table in the example the recursive children of Sally should be Sally–>David–>Lisa,John,Barbara,Phil
I want to get these in a table along with Employee Level .
Sally 1
David 4
Lisa 3
John 3
Barbara 3
Phil 3
Thanks!
I’ve tried the exact approach as mentioned in the blog but Inner join is not working as expected. After applying inner join between recursive query and CTE it is only showing result for anchor query. Please help
You probably joined on the EmpId instead of the Parent ID
Awesome work done
Hi, Thanks for a great explanation. I realize that this is an old thread and this has been asked before, but I did not see an answer. How would I then carry down the level 1 manager to all subsequent levels of employees so that I can see whom everyone rolls up to? the assumption being that there are multiple level 1 managers and each has a chain of command.
I am using Microsoft SQL Server Management Studio 2014 to export data. Some of the company names in my database have commas in the name. I am trying to export data into a true .csv file that surrounds each column in quotes. The option within SQL Server Management Studio 2014 doesn’t work. Do you have any suggestions to get around that?
I have also tried to create a .bat file, using the following line, but it is still not capturing the quotes around each column.
sqlcmd -S servername -d databasename -E -i “SQL_Statement.sql” -o “OutputFile.csv” -s”,” -W
Any help is greatly appreciated.
Thank you Pinal. You did simplified CONNECT BY VALUE and START WITH clauses of Oracle into SQL SERVER version.
I was thinking to use HierachyId::GetRoot() method. Thanks a lot. Pradyumna..
Hi, Thanks Dave.
My data has duplicate record in emp id. So when i used above query, i am getting below error. Can you please suggest
“The statement terminated. The maximum recursion 100 has been exhausted before statement completion”