Which is better? Performing calculations in sql or in your application

It depends on a lot of factors - but most crucially:

• complexity of calculations (prefer doing complex crunching on an app-server, since that scales out; rather than a db server, which scales up)
• volume of data (if you need to access/aggregate a lot of data, doing it at the db server will save bandwidth, and disk io if the aggregates can be done inside indexes)
• convenience (sql is not the best language for complex work - especially not great for procedural work, but very good for set-based work; lousy error-handling, though)

As always, if you do bring the data back to the app-server, minimising the columns and rows will be to your advantage. Making sure the query is tuned and appropriately indexed will help either scenario.

Re your note:

and then loop through the records

Looping through records is almost always the wrong thing to do in sql - writing a set-based operation is preferred.

As a general rule, I prefer to keep the database's job to a minimum "store this data, fetch this data" - however, there are always examples of scenarios where an elegant query at the server can save a lot of bandwidth.

Also consider: if this is computationally expensive, can it be cached somewhere?

If you want an accurate "which is better"; code it both ways and compare it (noting that a first draft of either is likely not 100% tuned). But factor in typical usage to that: if, in reality, it is being called 5 times (separately) at once, then simulate that: don't compare just a single "1 of these vs 1 of those".