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APPROX_COUNT_DISTINCT function in SQL Server

APPROX_COUNT_DISTINCT is an aggregate function in SQL Server which returns the approximate number of distinct values. This is very efficient with large number of rows. 

Syntax

APPROX_COUNT_DISTINCT (expression)

The return types of this function is bigint. And expression type can be of any type, except image, sql_variant, ntext, or text - as per Microsoft documentation. 

Let's look into an example,

SELECT APPROX_COUNT_DISTINCT([ProductID]) AS Approx_Distinct_Product
FROM [AdventureWorks2019].[Production].[WorkOrder];

The above statement returns the approximate number of different products from WorkOrder table.

APPROX_COUNT_DISTINCT

This is very powerful with billions of rows without any high memory utilization or performance issues.


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