Postgres 16

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
-- INIT database
CREATE TABLE city (name varchar(50) PRIMARY KEY, pop int);

CREATE TABLE

CREATE TABLE building (id SERIAL PRIMARY KEY, is_school boolean, city varchar(50));

CREATE TABLE

INSERT INTO city(name, pop) VALUES ('a', 10);

INSERT 0 1

INSERT INTO city(name, pop) VALUES ('b', 100);

INSERT 0 1

INSERT INTO city(name, pop) VALUES ('c', 1000);

INSERT 0 1

INSERT INTO building(is_school, city) VALUES (true, 'b');

INSERT 0 1

INSERT INTO building(is_school, city) VALUES (true, 'b');

INSERT 0 1

INSERT INTO building(is_school, city) VALUES (false, 'a');

INSERT 0 1

INSERT INTO building(is_school, city) VALUES (true, 'c');

INSERT 0 1

SELECT
  1.0 * sum(case when exists (
  select 1 from building b where b.city = c.name and is_school) then 1 else 0 end)
  / count(*) as p
FROM city c;

p

0.66666666666666666667

SELECT 1

select 1.0 * sum(coalesce(n, 0)) / count(*) as p
from city c
left join lateral (
  select 1 from building b where b.city = c.name and b.is_school limit 1
) x (n) on true

p

0.66666666666666666667

SELECT 1