which code table is for issuing certificate for example, the student has already made a certificate and the administrator sees the student has made a certificate and Moodle of the code, which is I need a certificate code in the database
When it comes to certificates, there is no core module, but one could install an addon. Examples:
https://docs.moodle.org/404/en/Certificate_module
https://moodle.org/plugins/mod_customcert
which would create new tables for the addons.
If we are looking the related tables, try this from mysql client using the DB for your moodle - the following is a guess (assuming 'cert' would be in the name of tables for such a mod/addon):
show tables like '%cert%';
'SoS', Ken