APPROX_COUNT_DISTINCT (Transact-SQL)

APPLIES TO: noSQL Server yesAzure SQL Database yesAzure SQL Data Warehouse yesParallel Data Warehouse

This function returns the approximate number of unique non-null values in a group.

Topic link icon Transact-SQL Syntax Conventions

Note

APPROX_COUNT_DISTINCT is a public preview feature.

Syntax

-- Syntax for Azure SQL Database, Azure SQL Data Warehouse and Parallel Data Warehouse  

APPROX_COUNT_DISTINCT ( expression )   

Arguments

expression
An expression of any type, except image, sql_variant, ntext, or text.

Return types

bigint

Remarks

APPROX_COUNT_DISTINCT( expression ) evaluates an expression for each row in a group, and returns the approximate number of unique non-null values in a group. This function is designed to provide aggregations across large data sets where responsiveness is more critical than absolute precision.

APPROX_COUNT_DISTINCT is designed for use in big data scenarios and is optimized for the following conditions:

  • Access of data sets that are millions of rows or higher and
  • Aggregation of a column or columns that have many distinct values

The function implementation guarantees up to a 2% error rate within a 97% probability.

APPROX_COUNT_DISTINCT requires less memory than an exhaustive COUNT DISTINCT operation. Given the smaller memory footprint, APPROX_COUNT_DISTINCT is less likely to spill memory to disk compared to a precise COUNT DISTINCT operation.

Note

With collation sensitive strings, the Public Preview version of APPROX_COUNT_DISTINCT uses a binary match and provides results that would have been generated in the presence of BIN collations and not BIN2.

Examples

A. Using APPROX_COUNT_DISTINCT

This example returns the approximate number of different order keys from the orders table.

SELECT APPROX_COUNT_DISTINCT(O_OrderKey) AS Approx_Distinct_OrderKey
FROM dbo.Orders;

Here is the result set.

Approx_Distinct_OrderKey
------------------------
15164704

B. Using APPROX_COUNT_DISTINCT with GROUP BY

This example returns the approximate number of different order keys by order status from the orders table.

SELECT O_OrderStatus, APPROX_COUNT_DISTINCT(O_OrderKey) AS Approx_Distinct_OrderKey
FROM dbo.Orders
GROUP BY O_OrderStatus
ORDER BY O_OrderStatus; 

Here is the result set.

O_OrderStatus                                                    Approx_Distinct_OrderKey
---------------------------------------------------------------- ------------------------
F                                                                7397838
O                                                                7387803
P                                                                388036

See also

Aggregate Functions (Transact-SQL)
COUNT (Transact-SQL)