Explanation:

1. Table Class:

   • This class represents a table in the database. It stores rows as a vector of unordered maps where the key represents column names, and the value represents the cell data.
   • It supports basic CRUD operations: insert, update, deleteRow, and select.

2. Database Class:

   • Manages a collection of tables. It provides methods to create tables, retrieve a table, and execute queries using a handler class.

3. QueryHandler and Derived Classes:

   • QueryHandler is an abstract base class that provides an interface for executing SQL-like queries.
   • Specific query types (Insert, Update, Delete, and Select) are implemented as subclasses of QueryHandler.

4. CRUD Operations:

   • Insert, update, delete, and select operations are demonstrated using respective query handlers that are executed on the Database instance.

More Description:

1. Classes:

   • Table Class: Represents a table in the database. It stores rows (data), and each row is a dictionary of key-value pairs (column name and value). Arrows from Table will point to Rows to show the data it holds.
   • Database Class: Manages multiple tables. There is an arrow from Database to Table indicating that a Database contains multiple Tables.
   • QueryHandler: This is an abstract class that defines a common interface for executing queries. It is a parent class for different types of queries (Insert, Update, Delete, Select).
   • InsertQueryHandler, UpdateQueryHandler, DeleteQueryHandler, SelectQueryHandler: These are derived classes of QueryHandler and represent the implementation of specific SQL queries. They interact with the Table class to perform CRUD operations.

2. Relationships:

   • The Database class contains multiple Table instances, so there’s a one-to-many relationship between Database and Table.
   • The QueryHandler class is the base class for all query types, and it interacts with the Table class to manipulate data.
   • Each specific QueryHandler (Insert, Update, Delete, Select) directly interacts with a Table to execute its corresponding operation.
   • Rows in the Table are represented as key-value pairs, showing that each row contains column names and values.

3. Flow of Operations:

   • Queries are created as specific QueryHandler objects (Insert, Update, Delete, or Select).
   • These QueryHandler objects execute the operations on a Table instance.
   • The Database executes the queries using the executeQuery() function.

System architecture and the flow of operations for the in-memory DBMS:-

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**Explanation **

• Database: Manages multiple Table instances. It has a map tables, which stores Table objects identified by their names. The Database class can create new tables and execute queries.

• Table: Each Table stores data as Rows (a vector of unordered maps where keys are column names and values are data entries). It supports operations like insert(), update(), and deleteRow() on the data.

• Rows: These represent the actual data in the table in a key-value pair format (column name: value).

• QueryHandler: An abstract base class that defines the interface for executing queries. The derived classes (InsertQueryHandler, UpdateQueryHandler, DeleteQueryHandler, and SelectQueryHandler) implement specific query logic.

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Explanation of Process Flow:

1. User Issues Query: A user or program issues a query such as INSERT, UPDATE, SELECT, or DELETE.

2. Database Receives the Query: The query is passed to the Database class. The Database class contains multiple Table objects.

3. Database Searches for Relevant Table: Based on the query, the database searches for the table (from the map) that the query will operate on.

4. QueryHandler Executes Query: The QueryHandler object, depending on the type of query (Insert, Update, Select, or Delete), will be created and executed.

5. QueryHandler Calls Specific Method: Each query handler calls the relevant method (e.g., insert(), update(), deleteRow(), or select()) on the corresponding Table.

6. Table Data is Manipulated: The Table class will perform the requested operation on its rows (data).

7. Table Returns Result: After the operation, the table either updates its data, returns the selected data, or confirms the row deletion.

8. Database Returns Response: Finally, the Database returns a response to the user, such as a success or error message.

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Ouput

Inserting Data into Employees Table...
Inserting Data into Departments Table...

Employees Table after Insertion:
id: 1    name: Alice    department: HR    
id: 2    name: Bob      department: IT    

Departments Table after Insertion:
id: 1    name: HR    
id: 2    name: IT    

Updating Data in Employees Table...

Employees Table after Update:
id: 1    name: Alice Updated    department: HR    
id: 2    name: Bob              department: IT    

Deleting Data from Employees Table...

Employees Table after Delete:
id: 1    name: Alice Updated    department: HR