Single-row subquery returns more than one row error for COUNT() function - oracle

Question

I am trying to understand why this would throw an error-
select count((select 1 br from dual
union
select 2 br from dual))
from dual;
ORA-01427: single-row subquery returns more than one row
But This won't-
select count(br)
from (select 1 br from dual
union
select 2 br from dual);
BR
----------
2

Answer 1

Take a look at the syntax of COUNT function here:
This function accepts the expression as it's argument.
Now take a look at the definition of the expression here.
An expression is a combination of one or more values, operators, and SQL functions that evaluates to a value.
A value - means single, one, scalar, individual, simple, atomic value
Not many values like a vector, an array, a table or a resultset.
Just one value, like 2 or 10.
The expression can have many forms, could be even a subquery, but SQL restricts this subquery to a special form called a scalar subquery, see this:
expression :=
A scalar subquery is defined here
A scalar subquery expression is a subquery that returns exactly one column value from one row.
Since the subquery in your query returns two rows, it doesn't conforms to the definition of the scalar subquery, and this is the reason of this error:
ORA-01427: single-row subquery returns more than one row
EDIT: Question #2: But isn't count supposed to count the number of rows in a result set?
Yes, of course. Count is one of aggregate functions.
See this chapter: http://docs.oracle.com/cd/E11882_01/server.112/e26088/functions003.htm#SQLRF20035
and also examples here: http://docs.oracle.com/cd/E11882_01/server.112/e26088/statements_10002.htm#SQLRF55380
aggregate functions return a single result row based on groups of rows, rather than on single rows. Aggregate functions can appear in select lists and in ORDER BY and HAVING clauses. They are commonly used with the GROUP BY clause in a SELECT statement, where Oracle Database divides the rows of a queried table or view into groups. In a query containing a GROUP BY clause, the elements of the select list can be aggregate functions, GROUP BY expressions, constants, or expressions involving one of these. Oracle applies the aggregate functions to each group of rows and returns a single result row for each group.
If you omit the GROUP BY clause, then Oracle applies aggregate functions in the select list to all the rows in the queried table or view. You use aggregate functions in the HAVING clause to eliminate groups from the output based on the results of the aggregate functions, rather than on the values of the individual rows of the queried table or view.
In simple words, aggregate functions can only be used in queries with GROUP BY and HAVING clauses, like this:
SELECT count( x )
FROM table
......
GROUP BY yyy
The special case is when you omit GROUP BY:
If you omit the GROUP BY clause, then Oracle applies aggregate functions in the select list to all the rows in the queried table or view.
SELECT count( x )
FROM table
But still, in these queries, count can accept only the expression (the scalar subquery) or * as it's argument.