CODEWITHSUNDEEP
sql-servercommon-table-expression
Sam Ccp
Sam Ccp

Reputation: 1162

CTE to get all children (descendants) of a parent

I've this problem that's giving me a headache...

Say, i've got a table with some thousands of rows and the structure of the table consists on a parent -> child relationship.

The relationships can go up to 6 levels. Here's a sample of the table structure:

ProductId | ParentId | Levels
1174           0        1174
311           1174      311, 1174
1186          311       1186, 311, 1174
448           1186      448, 1186, 311, 1174
3365          448       3365, 448, 1186, 311, 1174

We got a process that loops through the entire table to get the relationships and saves the "levels" column, this process is really slow ( because of the loops ) and i've tried with some cte to get the relationships but failed miserabily.

So far i've tried this cte but it doesn't do what i was hoping for and also, it seems to be replicating rows...

;With Parents(ProductId, ParentId, Levels)
As(
  Select ProductId, ParentId, Levels
  From Products
  Where ParentId = 0 
  Union All
  Select p.ProductId, p.ParentId, p.Levels
  From Products p
  Join Parents cte On cte.ProductId = p.ParentId
)
Select *
From Parents

As i mentioned early, we got a process that loops the table, it does its job but it can take up to 30 minutes, my question is is there a better way to do this? i know that CTE allows me to do it but i suck at it, also, the levels column should be calculated and updated on the table, is it possible?

Here's a Sqlfiddle in case someone could help, thanks!

Upvotes: 28

Views: 56714

Answers (2)

Charles Bretana
Charles Bretana

Reputation: 146499

;With Parents(ProductId, ParentId, Level, levels)
As(
  Select ProductId, ParentId, 0, 
     cast(ltrim(str(productId,8,0)) as varchar(max))
  From Products
  Where ParentId = 0 
  Union All
  Select p.ProductId, p.ParentId, 
      par.Level + 1,
      cast( levels + ', ' + ltrim(str(productId,8,0)) as varchar(max))
  From Products p
     Join Parents par
        On par.ProductId = p.ParentId
  )
  Select * From Parents
  Order By Level

Upvotes: 4

John Bustos
John Bustos

Reputation: 19544

This should do it:

WITH MyTest as
(
  SELECT P.ProductID, P.ParentID, CAST(P.ProductID AS VarChar(Max)) as Level
  FROM Products P
  WHERE P.ParentID = 0

  UNION ALL

  SELECT P1.ProductID, P1.ParentID, CAST(P1.ProductID AS VarChar(Max)) + ', ' + M.Level
  FROM Products P1  
  INNER JOIN MyTest M
  ON M.ProductID = P1.ParentID
 )
SELECT * From MyTest

And here's the updated SQL Fiddle.

Also, check out this link for help with CTEs... They're definitely good to know:

Hope this does the trick!

Upvotes: 57

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