using a function in where clause for SQL - mysql

Question

I'm trying to use a function in the where clause to get the previous business date in order to calculate the delta of a column in the table
I'm using the following query which isn't working:
select db1.x, db2.date, db1.date, db1.y as ValNow, db2.y as ValPrevious
from db db1, db db2
where db1.x = 275305
and db1.x = db2.x
and (dbo.getLastBusinessDate(db1.date) = db2.date)
order by db1.date
I need to use the function because it will include special dates where there are no values
EDIT:
Sorry for the vague message the error that I'm getting is
Msg 557, Level 16, State 2, Line 1
Only functions and some extended stored procedures can be executed from within a function.
I just want to display the values side by side right now to confirm that the sql statement is working correctly, I planned to use the following statement to calculate delta
select db1.date, db1.x, (db1.y - db2.y) as delta , DAY(db1.date), DAY(db2.date)
from
db db1
inner JOIN db db2 on db1.x = db2.x and dbo.getLastBusinessDate(db1.date) = db2.date
order by date desc

Answer 1

First you use LEFT JOIN because first date doesnt have "previous date". You may want handle that null.
Then in the join you use doble condition, first db1.x = db2.x and db1.date > db2.date to make sure you match an early date.
In the where you have to make sure you select the exact previous date.
This should work in both MySql or SqlServer, but in sql server you can use window function for a better result.
SELECT
db1.x,
db2.date,
db1.date,
db1.y as ValNow,
db2.y as ValPrevious
FROM
db db1
LEFT JOIN db db2
ON db1.x = db2.x
AND db1.date > db2.date
WHERE db1.x = 275305
AND not exist (SELECT db3.date
FROM db db3
WHERE db3.date > db2.date
AND db3.date < db1.date)
ORDER BY db1.date