仅基于SQL中的条件删除表中的重复项

Question

Let's say I have a table with columns:

CustomerNumber
Lastname
Firstname
PurchaseDate

...and other columns that do not change anything in the question if they're not shown here.

In this table I could have many rows for the same customer with different purchase dates (I know, poorly designed... I'm only trying to fix an issue for reporting, not really trying to fix the root of the problem).

How, in SQL, can I keep one record per customer with the latest date, and delete the rest? A group by doesn't seem to be working for my case

Answer 1

;with a as
(
select row_number() over (partition by CustomerNumber, Lastname, Firstname order by PurchaseDate desc) rn
from <table>
)
delete from a where rn > 1

Answer 2

This worked for me (on DB2):

 DELETE FROM my_table 
 WHERE (CustomerNumber, Lastname, Firstname, PurchaseDate) 
 NOT IN ( 
       SELECT CustomerNumber, Lastname, Firstname, MAX(PurchaseDate) 
       FROM my_table 
       GROUP BY CustomerNumber, Lastname, FirstName 
 )

Answer 3

SELECT CustomerNumber, Lastname, Firstname, MAX(PurchaseDate) LatestPurchaseDate
FROM Table
GROUP BY CustomerNumber, Lastname, Firstname

The MAX will select the highest (latest) date and show that date for each unique combination of the GROUP BY columns.

EDIT: I misunderstood that you wanted to delete records for all but the latest purchase date.

WITH Keep AS
(
    SELECT CustomerNumber, Lastname, Firstname, MAX(PurchaseDate) LatestPurchaseDate
    FROM Table
    GROUP BY CustomerNumber, Lastname, Firstname
)
DELETE FROM Table
WHERE NOT EXISTS
(
    SELECT *
    FROM Keep
    WHERE Table.CustomerNumber = Keep.CustomerNumber
    AND Table.Lastname = Keep.Lastname
    AND Table.Firstname = Keep.Firstname
    AND Table.PurchaseDate = Keep.LastPurchaseDate
)