SQLite computes the same answer as PostgreSQL here.
To be clear, I had to modify the code slightly to conform to PG syntax:
* Add a LIMIT 1 to the subquery. The LIMIT 1 is implied by SQLite, but
must be explicit in PG (also in MySQL and SQL Server).
* Change group_concat() into string_agg().
I end up with this:
> ~~~
CREATE TABLE t1(a varchar);
CREATE TABLE t2(b varchar);
INSERT INTO t1 VALUES(1);
INSERT INTO t2 VALUES(2),(3);
SELECT (SELECT string_agg(a,',') FROM t2 LIMIT 1) FROM t1;
~~~
To which PG gives an answer of just "1", same as SQLite.