MYSQL: COUNT() reference

In a nutshell

The COUNT() function counts the number of items in a result set.

Variations

COUNT(*)

Returns the number of rows in a table or group (when used with the GROUP BY clause). It includes rows with NULL values.

Example without GROUP BY to count total number of employees:

SELECT COUNT(*) FROM employees;

Example with GROUP BY to count the number of employees by department_id:

SELECT department_id, COUNT(*) as number_of_employees 
FROM employees 
GROUP BY department_id;

COUNT(column_name)

Returns the number of non-NULL values in the specified column. Rows where the specified column is NULL aren’t counted.

Example:

SELECT COUNT(employee_id) FROM employees;

COUNT(DISTINCT column_names)

Counts the number of distinct non-NULL values in the specified column. Useful to determine the number of unique values in the specified column

Example:

SELECT COUNT(DISTINCT department_id) FROM employees;