I have a MySQL database which contains quite a lot of records, and I'm writing a client to output a few of those records, selected by certain criteria, in a report.   Should I be filtering out the records that I need within the mysql, or within the client itself?

I'm often tempted to answer this question with another - "are you more comfortable writing SQL or (the client language)" ... then advising to use that. BUT ...

If you can do at least a preliminary select using a WHERE clause in your select, then you're saving a lot of traffic between your mysqld and client and that "has" to be more efficient.  Of course, if the tables are small it really doesn't make much difference, and if your selection criteria is exceptionally complex it might be that the select clause isn't going to have the "umph" to make the selection.

If your mysqld is being accessed from a different server, to the machine running the client, my laid back approach may change;  in that situation, you'll be sending a lot of traffic over your network which could have an effect ...

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