SQL Aggregate Function COUNT()

In SQL, the COUNT() function is an aggregate function that returns the number of rows that match a specified condition. The function takes a single argument, which is the name of the column or expression to be counted.

Here is an example of using the COUNT() function to count the number of rows in a employees table:

SELECT COUNT(*) AS num_employees FROM employees;

In this example, the COUNT() function is used to count the number of rows in the employees table. The AS keyword is used to alias the column name as num_employees.

The COUNT() function can also be used with the WHERE clause to count the number of rows that match a specified condition. For example, to count the number of employees with a salary greater than $50,000, we can use the following query:

SELECT COUNT(*) AS num_employees_above_50k FROM employees WHERE salary > 50000;

In this example, the COUNT() function is used with the WHERE clause to count the number of rows where the salary column is greater than $50,000. The result of the query is a single row with a single column, which contains the number of employees that meet the condition.

It's worth noting that COUNT() will include NULL values in the count, but there is also a variant called COUNT(column_name) which only counts non-NULL values in the specified column.