A few tips on grouping, counting or simply showing unique values.
Show a Unique List of Records
If there is a Table1 with users from different countries, represented in the columns UserID and Country, to list a distinct list of countries (the country will occur once only) - use DISTINCT.
SELECT DISTINCT
UserID
Country
FROM
Table1
Result
A list of countries or country codes, which you can use as a filter criteria.
Count UserIDs for Each Country
If there is a Table1 with users from different countries, represented in the columns UserID and Country, to list the countries and show the number of UserIDs for each country, use the aggregate function COUNT() and the GROUP BY statement.
SELECT
COUNT(UserID)
,Country
FROM
Table1
GROUP BY
Country
Result
A list of countries or country codes with a corresponding number of users for each.
Count and Sort Users for Each Country
If there is a Table1 with users from different countries, represented in the columns UserID and Country, to list the countries and show the number of UserIDs for each and then sort them, use the aggregate function COUNT(), the GROUP BY and the ORDER BY statement with the TOP clause.
SELECT TOP 500
COUNT(UserID)
,Country
FROM
Table1
GROUP BY
Country
ORDER BY
COUNT(UserID)
DESC