There simply is none. There's a key value store in LevelDB for the UTXO, and for miscellaneous things like the locations of block files on disk. Relational databses are far too slow for the task, and unnecessary given the type of information being stored.
some developers claim that they are gonna replace email with blockchain technology.
Blockchain is just a popular marketing buzzword. Its use is largely meaningless. You can safely ignore the marketing output of any person or business that are overly liberal with their use of buzzwords.
Wherever you see "blockchain" just substitute any of "nanotech", "...
It sounds like your wallet.dat file is still intact, but some of the environment data has gotten corrupted (possibly due to moving wallet files).
db_load -r lsn wallet.dat
This will reset the LSNs in the wallet.dat file which should cause the errors in the db.log to go away. Once you do that, delete the database/ folder and start Bitcoin Core.