How to use PostgreSQL bit_or function to aggregate column?

Question

I wish to use PostgreSQL's bit_or function on several rows of a single column of my table, but I can't figure out the proper usage for this.

Suppose I have bitwise flags in my column flags, and the rows of interest hold the numbers 1 (B00000001), 2 (B00000010), and 13 (B00001101). I want the output to be 15 (B00001111).

What I've tried so far is as follows:

SELECT bit_or(SELECT flags FROM items);
-- ERROR: syntax error at or near "select"
SELECT bit_or((SELECT flags FROM items));
-- ERROR: more than one row returned by a subquery used as an expression
SELECT bit_or(array(SELECT flags FROM items));
-- ERROR: function bool_or(integer[]) does not exist
SELECT bit_or(select array(SELECT flags FROM items));
-- ERROR: syntax error at or near "select"

Do you have any advice?

Answer 1

bit_or is an aggregate function, like sum or count.

SELECT bit_or(flags) FROM items;