Use cast('12345' as tab.col%type) as a substr - PLS-00220

Question

Hello,
I like to make sure that my constructed string fits into my variable, in a flexible way.
The variable is like
declare  t_comment tab.col%type; -- tab.col%type = varchar2(4)
and in the PLSQL I would like to do concat a string and make sure it fits in my variable, just cut the right end if it is too long. I want it to be dynamic, so NOT
t_comment := substr('1234567', 1, 4)
CAST () works like a substring operator if casting to a smaller data type, but also leads to hardcoding.
t_comment := cast ('1234567' as varchar2(4))
But I want
t_comment := cast ('1234567' as tab.col%type)
But that gives me a
PLS-00220: simple name required in this context.
And when I define a subtype
subtype comment_type is tab.col%type;
t_comment := cast ('1234567' as comment_type)
gives me a 
PLS-00382: expression is of wrong type
The documentation is also clear:
CAST converts one built-in datatype or collection-typed value into another built-in datatype or collection-typed value.
But it would be so nice to prevent hardcoding, to the varchar2(4) datatype in this case.
Any suggestions on how to make this flexible?

Answer 1

Hello,
you can read the column length from the data dictionary and then use it in your SUBSTR instead of the hard coded value:
SELECT data_length
INTO v_length
FROM user_tab_cols
WHERE table_name = 'TAB'
AND column_name = 'COL';
t_comment := SUBSTR('1234567', 1, v_length);
Not as simple as a %TYPE, but just as flexible.
Regards
Marcus