This chapter introduces various databases (relational and NoSQL) and related APIs. A relational database is a database that has a collection of tables containing data organized by the relations between data items. A relationship can be set up between each row in the table and a row in another table. Relationships may also be between columns inside a table (obviously columns within a table have to be related, for instance, a name column and an address column in a customer table) as well as connections between columns in other tables.
Not Only SQL (NoSQL) databases are frequently used in big data and web applications. NoSQL systems may permit SQL-like query languages to be employed. NoSQL databases allow data to be stored in a more flexible manner than the relational model permits. This may mean not having a database schema or a flexible database schema. Of course, the flexibility and speed may come at a price, such as limited support for consistent transactions...