Rules that Sub Queries must follow

Subqueries must be enclosed within parentheses.
A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns.
An ORDER BY cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY can be used to perform the same function as the ORDER BY in a subquery.
Subqueries that return more than one row can only be used with multiple value operators, such as the IN operator.
The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB.
A subquery cannot be immediately enclosed in a set function.
The BETWEEN operator cannot be used with a subquery; however, the BETWEEN operator can be used within the subquery.