Re: [SQL] sum-function

guenther@laokoon.IN-Berlin.DE (Christian Guenther) writes:
> udmsearch=> select sum( dict.word = 'mysql') from dict\g
> ERROR:  Unable to select an aggregate function sum(bool)
> What does the ERROR mean.

The '=' operator produces a boolean (true/false) result, but
sum() wants a numeric input.  Postgres is a strongly-typed
system, so it doesn't think booleans are interchangeable
with numerics.

It's been suggested several times that Postgres should
provide a standard conversion function that converts
boolean values to 0/1 numeric values, but no one's gotten
around to making one.  In the meantime you could get the
result that I think you want with something like

select sum(case when dict.word = 'mysql' then 1 else 0 end) ...
        regards, tom lane