A good introduction to Distinct for novices, though it it really doesnt explain why you would consider using a Group by to get a distinct list, is there a performance differential to consider when using one method or the other?

I also consider that these two SQL scripts are completely different and thus shouldnt be considered alternatives for each other

SELECT PostCode, COUNT(Postcode)

FROM

(

SELECT DISTINCT Address1, Address2, City, Postcode

FROM AddressTable

) AS Sub

GROUP BY Postcode

HAVING COUNT(Postcode) > 1

and

SELECT Address1,Address2,City,PostCode,Count(PostCode)

FROM AddressTable

GROUP BY Address1,Address2,City,PostCode

HAVING Count(PostCode) > 1

The first one would work on the following list of data

B23 3SP, 1

EC1A 3AD, 2.

which would result in only the EC1A 3AD, 2 row being returned, however the alternative query works on a totally different set of data, whch may be :

6 some road, some district, B23 6QD, 1

8 another road, another district, EC1A 3AD, 1

10 another road, another district, EC1A 3AD, 1

So with the Having Count()>1 clause you would get no results returned.