DATE arithmetic (DATEDIFF, DATE_ADD) in SQL - Mini Project: Build & Apply

Calculate Employee Contract Durations and Renewal Dates

📖 Scenario: You work in the HR department of a company. You have a table of employees with their contract start and end dates. You want to calculate how many days each employee's contract lasts and find the date when their contract should be renewed by adding 30 days to the end date.

🎯 Goal: Create SQL queries that calculate the duration of each employee's contract in days using DATEDIFF and find the renewal date by adding 30 days to the contract end date using DATE_ADD.

📋 What You'll Learn

Create a table called employees with columns id, name, contract_start, and contract_end.

Insert exactly three employees with specified contract start and end dates.

Write a query to calculate the contract duration in days using DATEDIFF.

Write a query to calculate the renewal date by adding 30 days to contract_end using DATE_ADD.

💡 Why This Matters

🌍 Real World

HR departments often need to track contract durations and renewal dates to manage employee agreements efficiently.

💼 Career

Knowing how to use date arithmetic functions like DATEDIFF and DATE_ADD is essential for database analysts and developers working with time-based data.

1

Create the employees table and insert data

Create a table called employees with columns id (integer), name (varchar), contract_start (date), and contract_end (date). Then insert these three employees exactly: (1, 'Alice', '2023-01-01', '2023-12-31'), (2, 'Bob', '2023-03-15', '2023-09-14'), and (3, 'Charlie', '2023-06-01', '2024-05-31').

SQL

-- Create the employees table and insert the three employees
-- Your code here

Need a hint?

Use CREATE TABLE to define the table and INSERT INTO to add the rows.

2

Add a helper column alias for contract duration calculation

Write a SELECT query from employees that includes all columns and add a new column alias called duration_days which will hold the number of days between contract_start and contract_end. Use DATEDIFF(contract_end, contract_start) for this calculation.

SQL

CREATE TABLE employees (
  id INT,
  name VARCHAR(50),
  contract_start DATE,
  contract_end DATE
);

INSERT INTO employees (id, name, contract_start, contract_end) VALUES
(1, 'Alice', '2023-01-01', '2023-12-31'),
(2, 'Bob', '2023-03-15', '2023-09-14'),
(3, 'Charlie', '2023-06-01', '2024-05-31');

-- Select all columns and add duration_days using DATEDIFF
-- Your code here

Need a hint?

Use Datediff(contract_end, contract_start) and alias it as duration_days.

3

Calculate contract renewal date by adding 30 days

Extend the previous SELECT query to add a new column alias called renewal_date which adds 30 days to the contract_end date. Use DATE_ADD(contract_end, INTERVAL 30 DAY) for this calculation.

SQL

CREATE TABLE employees (
  id INT,
  name VARCHAR(50),
  contract_start DATE,
  contract_end DATE
);

INSERT INTO employees (id, name, contract_start, contract_end) VALUES
(1, 'Alice', '2023-01-01', '2023-12-31'),
(2, 'Bob', '2023-03-15', '2023-09-14'),
(3, 'Charlie', '2023-06-01', '2024-05-31');

SELECT id, name, contract_start, contract_end, DATEDIFF(contract_end, contract_start) AS duration_days,
-- Add renewal_date column here using DATE_ADD
-- Your code here
FROM employees;

Need a hint?

Use DATE_ADD(contract_end, INTERVAL 30 DAY) and alias it as renewal_date.

4

Complete the query with ordering by renewal date

Modify the SELECT query to order the results by renewal_date in ascending order.

SQL

CREATE TABLE employees (
  id INT,
  name VARCHAR(50),
  contract_start DATE,
  contract_end DATE
);

INSERT INTO employees (id, name, contract_start, contract_end) VALUES
(1, 'Alice', '2023-01-01', '2023-12-31'),
(2, 'Bob', '2023-03-15', '2023-09-14'),
(3, 'Charlie', '2023-06-01', '2024-05-31');

SELECT id, name, contract_start, contract_end, DATEDIFF(contract_end, contract_start) AS duration_days,
       DATE_ADD(contract_end, INTERVAL 30 DAY) AS renewal_date
FROM employees
-- Add ORDER BY renewal_date ascending
-- Your code here

Need a hint?

Add ORDER BY renewal_date ASC at the end of the query.