--
-- test4.sql
--
create table t41(x1 int);
create table t42(x2 int);
--
select * from t41 where x1 = (select max(x2) from t42);
INFO:  analyzing "public.t41"
INFO:  "t41": scanned 0 of 0 pages, containing 0 live rows and 0 dead rows; 0 rows in sample, 0 estimated total rows
INFO:  analyzing "public.t42"
INFO:  "t42": scanned 0 of 0 pages, containing 0 live rows and 0 dead rows; 0 rows in sample, 0 estimated total rows
 x1 
----
(0 rows)

--
--
create table t43(x3 int);
create table t44(x4 int);
--
with v as (select x3 from t43) select count(*) from v where x3 = 123;
INFO:  analyzing "public.t43"
INFO:  "t43": scanned 0 of 0 pages, containing 0 live rows and 0 dead rows; 0 rows in sample, 0 estimated total rows
 count 
-------
     0
(1 row)

--
--
select 
 (select min(x3) from t43) as c1, 
 (select max(x4) from t44) as c2, 
 avg(x1) as c3 from t41;
INFO:  analyzing "public.t41"
INFO:  "t41": scanned 0 of 0 pages, containing 0 live rows and 0 dead rows; 0 rows in sample, 0 estimated total rows
INFO:  analyzing "public.t43"
INFO:  "t43": scanned 0 of 0 pages, containing 0 live rows and 0 dead rows; 0 rows in sample, 0 estimated total rows
INFO:  analyzing "public.t44"
INFO:  "t44": scanned 0 of 0 pages, containing 0 live rows and 0 dead rows; 0 rows in sample, 0 estimated total rows
 c1 | c2 | c3 
----+----+----
    |    |   
(1 row)