How can I know my workitem data is stored in which table of database?

As a user you have no business withthe DB. You can, if you have to access the API to access data. If needed, the reportable REST api https://jazz.net/help-dev/clm/index.jsp?topic=%2Fcom.ibm.team.reports.doc%2Ftopics%2Fr_ccm_rest_api.html could be used.

The application API is th e interface, not the DB.

HI Pravin,
As Ralph explained, database scheme is not disclosed for RTC application, but it is disclosed for data warehouse. If you are running data collection job (aka ETL job) for RTC, then you might check the data in data warehouse instead. For example, run the following SQL commands:

connect to DW
select NAME, NUM_VAL, VAL from RICALM.REQUEST_STRING_L_EXT

then you will see all list of Request Large String type of custom attributes.

For the detail, see
https://jazz.net/help-dev/clm/topic/com.ibm.team.reports.doc/topics/ccm/Custom-Attribute.html

Note that data warehouse is retrieving data by Reportable REST, so the result will be the same regardless of which way you verify the data.