In SQL, the COUNT() function is an aggregate function used to count the number of rows in a specified column or the number of non-NULL values in a set of rows. The COUNT() function can be used with the SELECT statement to retrieve the count of rows that match certain criteria.

Basic syntax

The basic syntax of the COUNT() function is as follows:

SELECT COUNT(column_name)
FROM table_name
WHERE condition

• COUNT(column_name): This counts the number of non-NULL values in the specified column_name. If no column name is provided inside the parentheses, COUNT(*) counts the total number of rows in the table.
• table_name: This is the name of the table from which you want to retrieve the data.
• condition: This is an optional condition that specifies which rows to include in the count.

Examples

1. Counting Total Number of Rows

To count the total number of rows in a table, you can use:

SELECT COUNT(*)
FROM employees

This query will return the total number of rows in the employees table.

1. Counting Number of Rows Based on a Condition

To count the number of employees who belong to the 'IT' department, you can use:

SELECT COUNT(*)
FROM employees
WHERE department = 'IT'

This query will return the count of rows where the department column has the value 'IT'.

1. Counting Number of Non-NULL Values in a Column

To count the number of non-NULL values in the salary column, you can use:

SELECT COUNT(salary)
FROM employees

This query will return the count of non-NULL values in the salary column.