Explain how you could make use of a sequence in your database. Provide code to create the sequence you propose and show the relevant output.Write an answer for 5 marks

Answer 1

A sequence in a database is a database object that generates a sequential list of unique numbers, which can be particularly useful for creating primary keys, unique identifiers, or timestamps for rows in a table. Using sequences helps ensure data integrity by preventing duplicate keys and can simplify the process of inserting new rows into a table.

Proposed Use Case:

Imagine a scenario where you have a table for customer orders. Each order needs a unique order ID, which could be efficiently managed using a sequence.

Steps to Create and Use a Sequence:

1. Create a Sequence: First, you will create a sequence that generates values for the order ID.
2. Use the Sequence: When inserting a new order into the orders table, you can utilize the sequence to assign a unique ID to the order.

Example Code:

Below is an example SQL code to create a sequence and use it in an INSERT statement. Assume we are using a PostgreSQL database.

-- Step 1: Create the sequence
CREATE SEQUENCE order_id_seq
    START WITH 1   -- Initial value of the sequence
    INCREMENT BY 1 -- Increment by 1 for each new value
    NO MINVALUE     -- No minimum value
    NO MAXVALUE     -- No maximum value
    CACHE 1;        -- Cache size

-- (Optional) Check the current value of the sequence
SELECT nextval('order_id_seq');

-- Step 2: Create an orders table
CREATE TABLE orders (
    order_id INT PRIMARY KEY, -- Will use the sequence to populate this
    customer_name VARCHAR(255),
    order_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

-- Step 3: Insert a new order using the sequence
INSERT INTO orders (order_id, customer_name) 
VALUES (nextval('order_id_seq'), 'John Doe');

-- Step 4 (Optional): Verify the inserted order
SELECT * FROM orders;

Relevant Output:

After executing the above code, you might have an output similar to:

 order_id | customer_name |       order_date        
----------+---------------+-------------------------
        1 | John Doe      | 2023-10-10 12:00:00

Explanation:

1. Create Sequence: The CREATE SEQUENCE statement initializes a sequence named order_id_seq.
2. Insert Statement: The INSERT INTO orders statement uses nextval() to get the next available value from the sequence for the order_id column.
3. Output Verification: The final SELECT statement retrieves the data from the orders table, showing that we successfully used the sequence to generate a unique order ID.

Conclusion:

Using a sequence simplifies the assignment of unique identifiers, ensures data integrity, and allows for easier management of primary keys in a relational database. This method is efficient and reduces the likelihood of key collisions during concurrent database operations.

Total: 5 Marks.