I have to migrate my custom system to typo3 version 6.2 but I did not find any database diagram(design/schema) of typo3. Would be great if someone can help. Thank you.
In a TYPO3 installation without extensions, there are not many tables that will interest you. Interesting tables are
pages and pages_language_overlay: Pages and their translations.tt_content: Content, translations are in the same table.sys_language: Define available languages. For pages with only a default language it can stay empty.sys_domain: Define Domains under which a site is available. For single domain installations it can be left empty.sys_file_reference: References to files, which are stored in table sys_files. The table sys_files will be automatically filled when you put files into the fileadmin folder.fe_users and fe_groups: Frontend users and groups.The other tables are either caching tables (names start with cf_) or are mostly used internally (most tables having names starting with sys_).
In some of the tables, especially in pages and tt_content, there are deprecated fields.
The usage of some fields depends on the actual TYPO3 setup. For example, the text of a content element from tt_content could be stored in the field bodytext, or in the field pi_flexform. So the question how to import your data depends on your setup.
For further information have a look at the official docs: https://docs.typo3.org/typo3cms/CoreApiReference/latest/ApiOverview/Database/DatabaseStructure/Index.html