Many thanks to everyone who responded to my question, it really is
I totally missed the HAVING clause in the SQL book I was looking at, and
the AS clause wasn't listed. Time to get a more up to date book I
I tried both methods and noticed that over a file with 4,000,000+
records the HAVING returned the first page of results in approximately
24 seconds whilst the AS only took around 2 seconds.
All the best and thanks again