How to get first_value from previous window partition - sql

Question

I want to display the the BalanceEndOfYesterday Value from the day before in a query as shown below.
| Date | Amout | BalanceEndOfDay | BalanceEndOfYesterday |
|------------|-------|-----------------|-----------------------|
| 2020-04-30 | 10 | 130 | 80 |
| 2020-04-30 | 20 | 130 | 80 |
| 2020-04-30 | 30 | 130 | 80 |
| 2020-04-30 | -10 | 130 | 80 |
| 2020-04-29 | 50 | 80 | 0 |
| 2020-04-29 | -10 | 80 | 0 |
| 2020-04-29 | 40 | 80 | 0 |
My query is
SELECT
BalanceEndOfDay ,
first_value(BalanceEndOfDay) OVER (ORDER BY Date DESC) -- here is some sort of window needed
FROM AccountTransactions

Answer 1

You can use apply :
SELECT at.*, COALESCE(at1.BalanceEndOfDay, 0) AS BalanceEndOfYesterday
FROM AccountTransactions at OUTER APPLY
( SELECT TOP (1) at1.BalanceEndOfDay
FROM AccountTransactions at1
WHERE at1.Date < at.Date
ORDER BY at1.Date DESC
) at1;
EDIT : If you want yesterday only balance then you can use dateadd() :
SELECT DISTINCT at.*, COALESCE(at1.balanceendofday, 0) AS BalanceEndOfYesterday
FROM AccountTransactions at LEFT JOIN
AccountTransactions at1
ON at1.date = dateadd(day, -1, at.date);

Answer 2

We could use LAG here, after first aggregating by date to obtain a single end of day balance for each date. Then, we can join your table to this result to pull in the end of day balance from yesterday.
WITH cte AS (
SELECT Date, MAX(BalanceEndOfDay) AS BalanceEndOfDay,
LAG(MAX(BalanceEndOfDay), 1, 0) OVER (ORDER BY Date) As BalanceEndOfYesterday
FROM AccountTransactions
GROUP BY Date
)
SELECT
a1.Date,
a1.Amount,
a1.BalanceEndOfDay,
a2.BalanceEndOfYesterday
FROM AccountTransactions a1
INNER JOIN cte a2
ON a1.Date = a2.Date
ORDER BY
a1.Date DESC;
Demo

Answer 3

If you want to do this using only window functions, you can use:
select at.*,
max(case when prev_date = dateadd(day, -1, date) then prev_BalanceEndOfDay end) over (partition by date) as prev_BalanceEndOfDay
from (select at.*,
lag(BalanceEndOfDay) over (order by date) as prev_BalanceEndOfDay,
lag(date) over (order by date) as prev_date
from accounttransactions at
) at;
Note: This interprets "the day before" as being exactly one day before. It is means "the day before in the data", then the first comparison should just be max(case when prev_date <> date . . . ).
Here is a db<>fiddle.
Note that in databases that fully support the range window specification, this can be done directly with logic like this:
max(BalanceEndOfDay) over (order by datediff(day, '2000-01-01', date)
range between 1 preceding and 1 preceding
)
Alas, SQL Server does not support this (standard) functionality.