SQL Server COUNT() Function

 The COUNT() function in SQL Server is an aggregate function used to count the number of rows that match a specified condition or all rows in a table. It is commonly used in SELECT queries to return the number of rows for a given condition.

Syntax:

SELECT COUNT(expression)
FROM table_name
WHERE condition;

• expression: The column or expression whose non-null values are counted.
• table_name: The table from which you want to count the rows.
• condition (optional): A condition to filter the rows.

Usage:

1. Count all rows in a table:

   SELECT COUNT(*)
   FROM employees;
   

   This query returns the total number of rows in the employees table.

2. Count rows with a specific condition:

   SELECT COUNT(*)
   FROM employees
   WHERE department = 'Sales';
   

   This query counts the number of rows in the employees table where the department column is 'Sales'.

3. Count non-NULL values in a specific column:

   SELECT COUNT(salary)
   FROM employees;
   

   This counts the number of non-NULL values in the salary column.

Notes:

• COUNT(*) counts all rows, including rows with NULL values.
• COUNT(column_name) counts only the rows where the specified column is not NULL.
• The WHERE clause can be used to filter rows based on specific conditions.