Question

Which is the proper way of checking events from current month on SQL Server and why?

1) WHERE (DATEDIFF(month, EventTime, GETDATE())=0))

2) WHERE (YEAR(EventTime) = YEAR(GETDATE()) AND MONTH(EventTime) = MONTH(GETDATE()))

Date format in table is i.e. EventTime: 2011-11-30 15:68:25.000

Answer 1

I don’t have access to a SQL Server with a profiler, so I can’t actually give as detailed an answer as I’d like.

The question is basically about which one allows the least calculation and most effective use of indexes.

Variants that use string manipulation have the highest calculation load, and don’t use indexes at all. So I’ll just skip those. That leave four common expressions…

SELECT * FROM date_sargable
WHERE  YEAR(value) = YEAR (getDate())
  AND MONTH(value) = MONTH(getDate())
;

SELECT * FROM date_sargable
WHERE DATEDIFF(MONTH, value, getDate()) = 0
;

SELECT * FROM date_sargable
WHERE DATEDIFF(MONTH, 0, value) = DATEDIFF(MONTH, 0, getDate())
;


SELECT * FROM date_sargable
WHERE value >= DATEADD(MONTH, DATEDIFF(MONTH, 0, getDate())    , 0)
  AND value <  DATEADD(MONTH, DATEDIFF(MONTH, 0, getDate()) + 1, 0)
;

The first three use INDEX SCANs, but the last one uses an INDEX SEEK. The difference is that the format of the query allows the optimiser to know you want a specific range of the data, that it’s all next to each other in one block of the index, and that it’s very easy to find that block.

If, when looking at execution plans, you see a SEEK in one version, and a SCAN in another, you’re much more likely to benefit from the SEEK.