How to add check for January in SQL statement - sql

Question

I'm creating a report using SQL to pull logged labor hours from our labor database for the previous month. I have it working great, but need to add logic to prevent it from breaking when it runs in January. I've tried adding If/Then statements and CASE logic, but I don't know if I'm just not doing it right, or if our system can't process it. Here's the snippet that pulls the date range:
SELECT
...
FROM
...
WHERE
...
AND
YEAR(ENTERDATE) = YEAR(current date) AND MONTH(ENTERDATE) = (MONTH(current date)-1)

Answer 1

Just use AND as a barrier like this. In January, the second clause will be executed instead of the first one:
SELECT
...
FROM
...
WHERE
...
AND
(
(
(MONTH(current date) > 1) AND
(YEAR(ENTERDATE) = YEAR(current date) AND MONTH(ENTERDATE) = (MONTH(current date)-1))
-- this one gets used from Feb-Dec
)
OR
(
(MONTH(current date) = 1) AND
(YEAR(ENTERDATE) = YEAR(current date) - 1 AND MONTH(ENTERDATE) = 12)
-- alternatively, in Jan only this one gets used
)
)

Answer 2

If your report is always going to be for the previous month, then I think the simplest idea is to declare the year and month of the previous month and then reference those in the Where clause. For example:
Declare LastMo_Month Integer = MONTH(DATEADD(MONTH,-1,getdate()));
Declare LastMo_Year Integer = YEAR(DATEADD(MONTH,-1,getdate()));
Select ...
Where MONTH(EnterDate) = #LastMo_Month
and YEAR(EnterDate) = #LastMo_Year
You could even take it a step further and allow the report to be created for any number of months ago:
Declare Delay Integer = -1;
Declare LastMo_Month Integer = MONTH(DATEADD(MONTH,#Delay,getdate()));
Declare LastMo_Year Integer = YEAR(DATEADD(MONTH,#Delay,getdate()));
Select ...
Where MONTH(EnterDate) = #LastMo_Month
and YEAR(EnterDate) = #LastMo_Year
Hope this helps.
PS - This is my first answer on StackOverflow, so sorry if the formatting isn't right!

Answer 3

if(month(getdate()) = 1)
begin
your jan logic
end
else
begin
your logic
end
The above answer with the Case is ok, but running a CASE on a huge result set would be pretty costly

Answer 4

WHERE
...
AND
DATEPART(yy,ENTERDATE) = DATEPART(yy,DATEADD(m,-1,ENTERDATE))
AND DATEPART(m,ENTERDATE) = DATEPART(m,DATEADD(m,-1,ENTERDATE))

Answer 5

Which Dialect of SQL are you speaking?
As opposed to doing it all with case statements, just use the built it date / time functions to subtract a month from the current date, which should handle crossing year boundaries.
TransACT
WHERE
YEAR(ENTERDATE) = year(dateadd(MONTH,-1, CURRENT_TIMESTAMP))
AND MONTH(ENTERDATE) = month(dateadd(MONTH,-1, CURRENT_TIMESTAMP))
Mysql
WHERE
YEAR(ENTERDATE) = YEAR(date_sub(curdate(),INTERVAL 1 MONTH))
AND MONTH(ENTERDATE) = MONTH(date_sub(curdate(),INTERVAL 1 MONTH) )

Answer 6

Try adding the previous month and year to your SELECT statement:
SELECT
...
,CASE MONTH(current date)
WHEN 1 THEN 12
ELSE MONTH(current date)-1
END AS previous_month
,CASE MONTH(current date)
WHEN 1 THEN YEAR(current date)-1
ELSE YEAR(current date)
END AS previous_year
FROM
...
WHERE
...
AND YEAR(ENTERDATE) = previous_year
AND MONTH(ENTERDATE) = previous_month
This should allow you to set the value before the WHERE comparison. This should be the most performant way to perform this procedure, as it avoids creating two entirely separate clauses or using OR.