I am just starting on Teradata and I have come across an Ordered Analytical Function called "Rows unbounded preceding" in Teradata. I tried several sites to learn about the function but all of them uses a complicated example explaining the same. Could you please provide me with a naive example so that I can get the basics clear?
It's the "frame" or "range" clause of window functions, which are part of the SQL standard and implemented in many databases, including Teradata.
A simple example would be to calculate the average amount in a frame of three days. I'm using PostgreSQL syntax for the example, but it will be the same for Teradata:
WITH data (t, a) AS (
VALUES(1, 1),
(2, 5),
(3, 3),
(4, 5),
(5, 4),
(6, 11)
)
SELECT t, a, avg(a) OVER (ORDER BY t ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
FROM data
ORDER BY t
... which yields:
t a avg
----------
1 1 3.00
2 5 3.00
3 3 4.33
4 5 4.00
5 4 6.67
6 11 7.50
As you can see, each average is calculated "over" an ordered frame consisting of the range between the previous row (1 preceding
) and the subsequent row (1 following
).
When you write ROWS UNBOUNDED PRECEDING
, then the frame's lower bound is simply infinite. This is useful when calculating sums (i.e. "running totals"), for instance:
WITH data (t, a) AS (
VALUES(1, 1),
(2, 5),
(3, 3),
(4, 5),
(5, 4),
(6, 11)
)
SELECT t, a, sum(a) OVER (ORDER BY t ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
FROM data
ORDER BY t
yielding...
t a sum
---------
1 1 1
2 5 6
3 3 9
4 5 14
5 4 18
6 11 29
Here's another very good explanations of SQL window functions.