SQL COUNT() Function

 The COUNT() function in SQL is used to return the number of rows in a result set. It is commonly used to count rows in a table, either including all rows or only those that match a specific condition.

Syntax

COUNT(expression)

Key Points:

• COUNT(*): Counts all rows in a table, including those with NULL values.
• COUNT(column_name): Counts only the non-NULL values in the specified column.
• COUNT(DISTINCT column_name): Counts the unique non-NULL values in the specified column.

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Examples

1. Count All Rows

SELECT COUNT(*) AS TotalRows
FROM Employees;

Output: Total number of rows in the Employees table.

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2. Count Non-NULL Values in a Column

SELECT COUNT(Salary) AS TotalSalaries
FROM Employees;

Output: Counts the number of non-NULL values in the Salary column.

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3. Count Unique Values

SELECT COUNT(DISTINCT Department) AS UniqueDepartments
FROM Employees;

Output: Counts the distinct number of departments.

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4. Count Rows with a Condition

SELECT COUNT(*) AS HighEarningEmployees
FROM Employees
WHERE Salary > 50000;

Output: Counts the number of employees earning more than 50,000.

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Use with GROUP BY

The COUNT() function is often used with the GROUP BY clause to count rows for each group.

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

Output: The count of employees in each department.