q1: given a table : ID, Date, Status. Write a SQL query which receives @ID @Date and retrieve the status the person with that id on that date or the last status before that date if that date doesn't exist.

q2: given a table : ID, Date, Money. Write a SQL query which retriev开发者_如何学Pythone the average money amount for the dates that appears in the table.

i.e. for the following table the average is 600/2 (there are two different dates)

ID1    1/1/2010  100
ID2    1/1/2010  300
ID2    2/1/2010  200

I have tried to write queries for above questions. I would appreciate you remarks:

Answer 1

SELECT TOP 1 status from MyTable AS T
WHERE T.Date <= @date && T.id = @id
ORDER BY T.Date

Answer 2

declare money_sum decimal
declare days_count  int
money_sum =
SELECT SUM(MONEY) FROM table_name

days_count =
SELECT DISTINCT COUNT(date) FROM table_name

return money_sum/ days_count

The syntax, would depend on the rdbms you're using, i'm assuming MSSQL, or possibly sybase, based on the TOP syntax you've used, in which case, you'll need to use @ in front of your variables, and the set keyword:

declare @money_sum decimal
declare @days_count  int

set @money_sum = SELECT SUM(MONEY) FROM table_name
set @days_count = SELECT COUNT(DISTINCT date) FROM table_name

return @money_sum/ @days_count

Notice how i also moved the distinct inside the count function.