SQL Server 2019 (Linux)

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select @@version;

(No column name)

Microsoft SQL Server 2019 (RTM-CU32-GDR) (KB5102335) - 15.0.4480.2 (X64) 
	Jun 18 2026 17:54:32 
	Copyright (C) 2019 Microsoft Corporation
	Express Edition (64-bit) on Linux (Ubuntu 20.04.6 LTS) <X64>

-- APPROX_COUNT_DISTINCT arrived in SQL Server 2019
create table hit (visitor int);
insert into hit values (1),(1),(2),(3),(3),(4);

6 rows affected

select count(distinct visitor)        as exact
     , approx_count_distinct(visitor)  as approximate
from hit;

exact
approximate

4
4