What is Using DISTINCT with aggregate functions

You should be aware of the use of DISTINCT in a SELECT clause to remove duplicate rows. When used with an aggregate function, DISTINCT removes duplicate values from the input column before computing the summary value. DISTINCT is useful when summarizing unique occurrences of values, such as customers in the orders table.

The following example returns the number of customers who have placed orders, no matter how many orders they placed:

SQLCopy

SELECT COUNT(DISTINCT CustomerID) AS UniqueCustomers
FROM Sales.SalesOrderHeader;

COUNT(<some_column>) merely counts how many rows have some value in the column. If there are no NULL values, COUNT(<some_column>) will be the same as COUNT(*). COUNT (DISTINCT <some_column>) counts how many different values there are in the column.