Deleting or Changing Records in ETL 2: A Deep Dive

In this post, we’ll explore the intricacies of deleting or changing records in a table using ETL (Extract, Transform, Load) techniques. We’ll dive into the details of the provided SQL query and discuss how to modify it to achieve our desired outcome.

Background Information

ETL is a common data integration technique used in various industries to extract data from multiple sources, transform it into a standardized format, and load it into a target system. In this context, we’re dealing with a table that contains records with labels, costs, and timestamps.

The provided SQL query uses several window functions to achieve the desired outcome. Let’s break down each function used in the query:

ROW_NUMBER(): assigns a unique number to each record within a partition of a result set.
LAG() : returns the value of a column from a previous row in the same partition.
COUNT(*) OVER (PARTITION BY label) as cnt: calculates the total count of records for each label.
RANK() : assigns a rank to each record based on a specific column.

The Original Query

The original query is designed to delete or change records from the table where there are duplicate entries with the same label and cost. However, it doesn’t account for cases where the records have different timestamps, leading to unexpected results.

; with todelete as (
      select *, 
           count(*) over (partition by label) as cnt, 
     lag(cost) over (partition by label order by time asc) as lastcost
     ,ROW_NUMBER() over (partition by label order by time ASC) as r_number
    from Table1
)
DELETE from todelete 
    where cnt > 1 and r_number between 1 and (cnt/2)*2 and cost=ISNULL(lastcost,cost)

The Issue with the Original Query

The original query deletes both records for a label when there are duplicates. However, we want to delete only the older record.

To achieve this, we need to modify the query to account for the timestamp column. We can use a combination of window functions and conditions to identify the older record.

Modifying the Query

Let’s introduce two additional ranking functions: RANK() and ROW_NUMBER(). The first function assigns a rank based on the cost and time, while the second function assigns a row number based on the label and timestamp in descending order.

; with todelete as (
      select *, 
           count(*) over (partition by label) as cnt, 
     lag(cost) over (partition by label order by time asc) as lastcost
     ,ROW_NUMBER() over (partition by label order by time ASC) as r_number
    ,RANK() over (partition by cost order by time asc) as TEST
    ,Row_NUMBER() over (partition by label order by TIME DESC) as TEST2
       from Table1
     )
DELETE from todelete 
    where (cnt > 1 and r_number between 1 and (cnt/2)*2 and cost=ISNULL(lastcost,cost) AND TEST2 !=1)  OR (cnt&gt;1 AND TEST2&lt;&gt;1 AND r_number2 != 1)

Explanation

The modified query uses two conditions to identify the older record:

TEST2 !=1: checks if the rank of the current record is not equal to 1, indicating that it’s an older record.
r_number2 != 1: checks if the row number of the current record is not equal to 1, also indicating that it’s an older record.

The query uses these conditions in conjunction with the existing conditions to ensure that only the older record is deleted or changed.

Example Use Cases

Here are some example use cases for this modified query:

Delete both records for a label when there are duplicates and no timestamp difference.

; with todelete as (
      select *, 
           count(*) over (partition by label) as cnt, 
     lag(cost) over (partition by label order by time asc) as lastcost
     ,ROW_NUMBER() over (partition by label order by time ASC) as r_number
    from Table1
)
DELETE from todelete 
    where cnt > 1 and r_number between 1 and (cnt/2)*2 and cost=ISNULL(lastcost,cost)

Delete the older record when there are duplicates with different timestamps.

; with todelete as (
      select *, 
           count(*) over (partition by label) as cnt, 
     lag(cost) over (partition by label order by time asc) as lastcost
     ,ROW_NUMBER() over (partition by label order by time ASC) as r_number
    ,RANK() over (partition by cost order by time asc) as TEST
    ,Row_NUMBER() over (partition by label order by TIME DESC) as TEST2
       from Table1
     )
DELETE from todelete 
    where (cnt > 1 and r_number between 1 and (cnt/2)*2 and cost=ISNULL(lastcost,cost) AND TEST2 !=1)  OR (cnt&gt;1 AND TEST2&lt;&gt;1 AND r_number2 != 1)

By using this modified query, you can effectively delete or change records in a table while ensuring that only the older record is processed.

Last modified on 2024-01-07