What is SQL?

In this table, each row represents an employee, and each column represents a particular attribute of the employee.

SQL allows us to retrieve and analyze this information easily.

SELECT *
FROM Employees;

This query retrieves all columns and records from the Employees table.

To retrieve only employee names and salaries:

SELECT EmployeeName, Salary
FROM Employees;

2. WHERE Clause

The WHERE clause filters records based on a specified condition.

Example:

SELECT *
FROM Employees
WHERE Salary > 40000;

This query returns employees whose salary is greater than 40,000.

3. ORDER BY Clause

The ORDER BY clause sorts records in ascending or descending order.

Example:

SELECT *
FROM Employees
ORDER BY Salary DESC;

This query displays employees from the highest salary to the lowest salary.

4. GROUP BY Clause

The GROUP BY clause groups records that have the same values in specified columns. It is commonly used with aggregate functions.

Example:

SELECT Department, COUNT(*) AS TotalEmployees
FROM Employees
GROUP BY Department;

This query calculates the total number of employees in each department.

5. Aggregate Functions

SQL provides aggregate functions to summarize data.

Common aggregate functions include:

• COUNT() – Counts records.

• SUM() – Calculates the total.

• AVG() – Calculates the average.

• MIN() – Returns the minimum value.

• MAX() – Returns the maximum value.

Example:

SELECT
    COUNT(*) AS TotalEmployees,
    AVG(Salary) AS AverageSalary,
    MAX(Salary) AS HighestSalary
FROM Employees;

This query calculates the total number of employees, average salary, and highest salary.

6. JOIN Statement

A JOIN combines related records from two or more tables based on a matching column or condition.

For example, an Employees table may contain DepartmentID, while a Departments table contains department names.

SELECT
    E.EmployeeName,
    D.DepartmentName
FROM Employees AS E
INNER JOIN Departments AS D
    ON E.DepartmentID = D.DepartmentID;

This query returns employees along with their matching department names.

7. INSERT Statement

The INSERT statement adds new records to a table.

Example:

INSERT INTO Employees
    (EmployeeID, EmployeeName, Department, Salary)
VALUES
    (105, 'Rohit', 'Finance', 40000);

This query inserts a new employee record into the Employees table.

8. UPDATE Statement

The UPDATE statement modifies existing records.

Example:

UPDATE Employees
SET Salary = 48000
WHERE EmployeeID = 101;

This query changes the salary of the employee whose EmployeeID is 101.

Important: Always check the WHERE condition before running an UPDATE statement.

9. DELETE Statement

The DELETE statement removes records from a table.

Example:

DELETE FROM Employees
WHERE EmployeeID = 105;

This query deletes the employee record with EmployeeID 105.

Important: Without a WHERE condition, a DELETE statement can remove all records from the table.