Math Operations - DB or App?

i'm in the process of developing some reports for an
application i'm developing.

i'm curious how the more experienced developers do
their math operations.  do you do it in your select or
do you do it in your app?

i can't give an example within a select (that's what
i'm researching next).

an example of doing the math in the app (php) would be
getting a value $p for number of passed units and a
value $t for total units and then setting yield to $y
= $t/$p.

what do you do and why?  what are the pluses and
minuses of each approach?

as always, tia...

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