data-konzept

'In this article the COUNT function will be viewed from three aspects:

• The different ways of using COUNT

• The potential performance issues related with COUNT

• COUNT usage as an aggregated function (demo example)

COUNT is among the most used functions in T-SQL codes. Because of that fact, it must be well understood by developers. COUNT returns the number of the items/rows in a group. COUNT works like the COUNT_BIG function. The only difference between the two functions is their return values.'...

http://www.sqlservercentral.com/articles/T-SQL/142568/