SQL DISTINCT
查詢不重複值
DISTINCT 用於去除重複資料，常用在國家、城市、分類等欄位。它很適合先快速理解資料有哪些分類值。
Example 1 - 單欄位 DISTINCT
SELECT DISTINCT Country FROM Customers;
Example 2 - 依城市去重複
SELECT DISTINCT City FROM Customers ORDER BY City;
Example 3 - 多欄位組合去重複
SELECT DISTINCT Country, City FROM Customers ORDER BY Country, City;
Example 4 - 計算不重複國家數
SELECT COUNT(DISTINCT Country) AS CountryCount FROM Customers;