Question

My question is similar to sql statement to delete records older than XXX as long as there are more than YY rows, but that question just deals with a single parent, I want to delete records for all parents in one go.

Consider this table:

CREATE TABLE Children
(
    ChildId int NOT NULL,
    ChildCreated datetime NOT NULL,
    ParentId int NOT NULL
) 

This could be any parent-child relationship, so the names are generic.

I would like to delete all children that are older than a month, but need to keep a minimum number of children for each parent regardless of their age.

I tried some statements with nested SELECTs and GROUP BYs which gave me some results but none gave me the correct result set.

Because I am using SQL Server I came up with the following solution that works great:

WITH CTE AS
(
  SELECT ROW_NUMBER() OVER (Partition BY ParentId ORDER BY ChildCreated DESC) 
  As RowNo, ChildCreated FROM Children
)

DELETE FROM CTE WHERE RowNo > 10
AND RevisionCreated < DATEADD(MONTH,-1,GetDate())

The common table expression groups all children for each parent together and adds a continuous row number based on the creation order. The newest child for each parent has
a row number of 1, the tenth newest has 10. So I can just delete all records with a row
number greater 10 as long as they are also over a month old.

My question is, what if I have to do the same thing on a system where CTEs are not supported. What is the ANSI SQL-92 solution for this problem?

Answer 1

Based on other responses, and the relative simplicity of my query I think I might be over simplifying the issue, but I am assuming since parentID is not nullable that it does not reference childID, In which case it can be achieved as simply as the below

DELETE  Children
FROM    Children a
WHERE   ChildCreated < DATEADD(MONTH, -1, GETDATE())
AND     (   SELECT  COUNT(*)    -- NUMBER OF NEWER CHILDREN WITH THE SAME PARENT
            FROM    Children b
            WHERE   a.ParentID = b.ParentID
            AND (   a.ChildCreated < b.ChildCreated
                OR  (a.ChildCreated = b.ChildCreated AND a.ChildID > b.ChildID)
                )
        ) >= 10

Although this exact SQL may need tweaking depending on RDBMS, I don’t know of any RDBMS where this principal cannot be applied.