Oracle 11g:Scalar Subquery expression Limitation

Scalar subqueries are not valid expressions in the following places: - As default values for columns - As hash expressions for clusters - In the RETURNING clause of DML statements - As the basis of a function-based index - In CHECK constraints - In GROUP BY clauses - In statements that are unrelated to queries, such as CREATE PROFILE

Reference:http://docs.oracle.com/cd/B28359_01/server.111/b28286/expressions013.htm