A blockchain is a read- and append-only storage methodology. This means that blocks can only be created and read in the blockchain ledger. Blocks in a blockchain cannot be updated or deleted; blocks can only be appended to the end of a blockchain. There is no access control in a public blockchain as it is open for both read and write operations.
On the other hand, relational databases follow the create, read, update, and delete (CRUD) operation model. Unlike the case with a blockchain, each database has an administrator when it is created, and they will assign access control to the other users. Relational databases are mostly maintained by a single entity who is in control of all the application data, whereas blockchain technology was designed for decentralized applications.
Figure 1.1 should help you to visualize the difference between a centralized database architecture and a blockchain architecture:
