General SQL question

Question

I have seen the following in some sql, but not sure of the difference.
select abc, NULL from xyz where .....
select abc, to_char(NULL) from xyz where ....
my question is there a difference between NULL and TO_CHAR(NULL) ?
Thanks.

Answer 1

BluShadow wrote:
Looks like someone was trying to cast the null to a varchar2 datatype so that SQL knew the datatype of the column. In that case they should have used the CAST function...
Blu:
Just out of curiosity, any particular reason for the preference for cast instead of to_datatype(null)? I often use that construct, particularly in union type queries. The cast is sematically meaningless in a union query since the column will get the size of the largest column in the query.
SQL> set null null
SQL> select *
  2  from (select 1 id, null c1, to_char(null) c2, cast(null as varchar2(10)) c3 from dual
  3        union all
  4        select 2, 'Hello there', 'Hello there', 'Hello there' from dual);
        ID C1          C2          C3
         1 null        null        null
         2 Hello there Hello there Hello there
SQL> create view v as
  2  select *
  3  from (select 1 id, null c1, to_char(null) c2, cast(null as varchar2(10)) c3 from dual
  4        union all
  5        select 2, 'Hello there', 'Hello there', 'Hello there' from dual);
View created.
SQL> desc v;
Name               Null?    Type
ID                          NUMBER
C1                          VARCHAR2(11)
C2                          VARCHAR2(11)
C3                          VARCHAR2(11)About the only place where I would use an explicit cast is if I was "hiding" the content of a column in a view, but needed to maintain the same structure as the table.
John