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Oracle sql query problem without to_number fuction

GouthamAnand asked
Last Modified: 2021-04-21
when I execute the below query I am not getting any rows.

SELECT count(*)
           FROM table_1
          WHERE EXISTS (
                   SELECT *  FROM table_2
                    WHERE part_num = SUBSTR (epart_num, 3, 8) )

but when I execute the below qry I am getting rows. The only diff is I put TO_NUMBER function to epart_num column.
Can you please let me know why I need to put TO_NUMBER function?
All columns in table_1 and table_2 are VARCHAR2 type only.

SELECT count(*)
           FROM table_1
          WHERE EXISTS (
                   SELECT *  FROM table_2
                    WHERE part_num = TO_NUMBER(SUBSTR (epart_num, 3, 8)) )

Also if I give
SELECT *  FROM table_2
                    WHERE part_num =123
then its throwing invalid number exception. If so how its working above after converting with to_number without thowing invalid number error rather its returning only with to_number and not working without to_number fuction.
Can anybody pls. tell?

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