ORA-06502 assigning values from SQL to PL/SQL variable

The results returned by the recent SQL query cause a ORA-06502 error to the PL/SQL variable. The error description is ORA-06502: PL/SQL: numeric or value error: character string buffer too small. obviously, the length of the character variable definition is not enough. It is added to 20 to 100. Continue to 06502. What is the problem?

1. Problem Description
-- The problem is that in a package, there are two parameter cursors, one parent cursor and one child cursor. When the output result of the parent cursor is passed to the child game tag, the system prompts that the value is too large.
-- The original SQL statement of the parent cursor is long and complex. to simplify the description, construct the environment

--> Oracle version
Goex_admin @ CNMMBO> select * from v $ version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Release 10.2.0.3.0-64bit Production
PL/SQL Release 10.2.0.3.0-Production
CORE 10.2.0.3.0 Production
TNS for Linux: Version 10.2.0.3.0-Production
NLSRTL Version 10.2.0.3.0-Production

--> Create a test table t and insert 3 Records
Goex_admin @ CNMMBO> create table t (dt char (8 ));

Table created.

Goex_admin @ CNMMBO> insert into t select '000000' from dual;

1 row created.

Goex_admin @ CNMMBO> insert into t select '000000' from dual;

1 row created.

Goex_admin @ CNMMBO> insert into t select '000000' from dual;

1 row created.

Goex_admin @ CNMMBO> commit;

Commit complete.

--> Use the following query output results to report a ora-06502 Error
--> The query statement is also relatively simple. Take the minimum value of the dt column of Table t and assign the value to the variable in the outer query.
--> Sub-queries on the outer layer seem to be superfluous, purely simulating the original environment.
Goex_admin @ CNMMBO> DECLARE
2. tradedate_out CHAR (100 );
3 BEGIN
4 SELECT tradedate
5 INTO tradedate_out
6 FROM (select min (dt) AS tradedate FROM t) d;
7
8 DBMS_OUTPUT.Put_Line ('trade _ date = '| tradedate_out );
9 END;
10/
DECLARE
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character string buffer too small
ORA-06512: at line 4

2. Rewrite Query
--> No similar error occurs if we remove the outer query.
--> In this case, does the value cause a variation when the child queries the outer layer?
Goex_admin @ CNMMBO> DECLARE
2. tradedate_out CHAR (100 );
3 BEGIN
4 select min (dt) INTO tradedate_out FROM t;
5
6 DBMS_OUTPUT.Put_Line ('trade _ date = '| tradedate_out );
7 END;
8/
Trade _ date = 20121218

PL/SQL procedure successfully completed.

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