Treat a subquery like a value

Postgres is cool with returning a value from a query.

select 1;

And also returning a value from a subquery.

select (select 1);

Turtles all the way down.

select (select (select 1));

But that subquery can only return one row or you'll get an error.

select (select unnest('{1, 2}'::integer[]));
ERROR:  more than one row returned by a subquery used as an expression

This is used sometimes when you need to use a count in an expression.

select round((select count(*) from pg_class where reltype = 0) / (select count(*)::float from pg_class) * 100) as percent_with_zero_reltype;
 percent_with_zero_reltype 
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