在表中有效查找最后日期-Teradata SQL

Question

Say I have a rather large table in a Teradata database, "Sales" that has a daily record for every sale and I want to write a SQL statement that limits this to the latest date only. This will not always be the previous day, for example, if it was a Monday the latest date would be the previous Friday.

I know I can get the results by the following:

SELECT s.*
FROM Sales s
JOIN (
      SELECT MAX(SalesDate) as SalesDate 
      FROM Sales
) sd 
ON s.SalesDate=sd.SalesDt

I am not knowledgable on how it would process the subquery and since Sales is a large table would there be a more efficient way to do this given there is not another table I could use?

Answer 1

That is probably fine, if you have an index on salesdate .

If there is only one row, then I would recommend:

select top 1 s.*
from sales s
order by salesdate desc;

In particular, this should make use of an index on salesdate .

If there is more than one row, use top 1 with ties .

Answer 2

Another (more flexible) way to get the top n utilizes OLAP-functions:

SELECT *
FROM Sales s
QUALIFY
  RANK() OVER (ORDER BY SalesDate DESC) = 1

This will return all rows with the max date. If you want only one of them switch to ROW_NUMBER.