Can there be a database-agnostic SQL query to fetch top N rows?

Question

We want to be able to select top N rows using a SQL Query. The target database could be Oracle or MySQL. Is there an elegant approach to this? (Needless to say, we\'re dealing w

Answer 1

If there is a unique key on the table yes...

Select * From Table O
Where (Select Count(*) From Table I
       Where [UniqueKeyValue] < O.UniqueKeyValue)  < N

You can substitute your own criteria if you want the "Top" definition to be based on some other logic than on the unique key...

EDIT: If the "sort" that defines the meaning of "Top" is based on a non-unique column, or set of columns, then you can still use this, but you can't guarantee you will be able to get exactly N records out...

  Select * From Table O
  Where (Select Count(*) From Table I
         Where nonUniqueCol < O.nonUniqueCol) < 10

If records 8, 9, 10, 11, and 12 all have the same value in [nonUniqueCol], then the query will either only generate 7 records, (with '<') ... , or 12 (if you use '<=')

NOTE: As this involves a correlated sub-query, the performance can be an issue for very large tables...