To cast or not to cast?

Question

In SQL, when one refers to another table to display a value (e.g. a field: (select something from a_table) column_alias) the engine gives to the column an unlimited range of the datatype.
For example when I 'm getting a varchar2 value it will show up as varchar2(4000).
So, most of the time if I know the limit of the values I get, I use the following:
cast ( (select something from a_table) as varchar2(100) ) column_alias
To me, this has the benefit of not having to limit the datatype when using application development tools (Forms and Reports in my case).
The question is: Does it really matter (is it beneficial) when it comes to database memory issues to use this manner?
Thanks in advance.

Answer 1

MichaelTsilikidis wrote:
In SQL, when one refers to another table to display a value (e.g. a field: (select something from a_table) column_alias) the engine gives to the column an unlimited range of the datatype.
For example when I 'm getting a varchar2 value it will show up as varchar2(4000).
Are you sure about that? Can you demonstrate your claim with an example? Below is an example which says otherwise.
SQL> desc emp;
Name                 Null?    Type
EMPNO                NOT NULL NUMBER
ENAME                         VARCHAR2(6)
JOB                           VARCHAR2(9)
MGR                           NUMBER
HIREDATE                      DATE
SAL                           NUMBER
COM                           NUMBER
DEPTNO                        NUMBER
ID                            NUMBER(10)
SQL> create table t
  2  as
  3  select (select ename from emp where mgr is null) ename
  4    from dual;
Table created.
SQL> desc t;
Name                 Null?    Type
ENAME                         VARCHAR2(6)
SQL>