Building a simple relational database from scratch involves implementing some of key concepts we've discussed in the previous sections. For our example, we will use Python to build a simple in-memory relational database that supports basic operations like writing, reading and updating data.

First, let's start by creating a Dictionary that will serve as our database. In Python, Dictionaries are a data structure similar to hash tables where every entry is a key-value pair. Thus, they are a perfect option to implement a simple key-value store.

Next, we'll define functions to handle the basic operations.

• write(key, value) function to store data: This function will write the data into the dictionary. The 'key' will serve as the unique identifier for the data, sort of like a primary key in a relational database.

• read(key) function to fetch data: This function will be used to retrieve data from the dictionary using a key. It's equivalent to a SQL's SELECT statement.

• update(key, new_value) function to update data: This function will take a key and new_value and change the existing value of the given key. This is similar to SQL's UPDATE command.

• delete(key) function to remove data: This function will remove a key-value pair from the dictionary. This is similar to SQL's DELETE command.

Remember that this is a very basic implementation. Although it lacks features of a fully-fledged relational database, it gives us an idea of how data can be stored and manipulated in a database. At a higher level, real-world databases include additional components like query optimizers, transaction logs, and background cleanup processes, but these basics apply.

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if __name__ == "__main__":

  # Initialize an empty dictionary to be our database

  db = {}

​

  # Define our functions

  def write(key, value):

    db[key] = value

​

  def read(key):

    return db.get(key, 'Not Found')

​

  def update(key, new_value):

    if key in db:

      db[key] = new_value

​

  def delete(key):

    if key in db:

      del db[key]

​

  # Let's test our database

  write('name', 'Alan Turing')

  print(read('name'))  # Outputs: Alan Turing

  update('name', 'Grace Hopper')

  print(read('name'))  # Outputs: Grace Hopper

  delete('name')

  print(read('name'))  # Outputs: Not Found

​

print('Program ended')