select distinct

在表中，一个列可能会包含多个重复值，有时您也许希望仅仅列出不同（distinct）的值。
distinct 关键词用于返回唯一不同的值。

SELECT DISTINCT column_name FROM table_name;
SELECT DISTINCT column_name FROM table_name;
SELECT DISTINCT column_name FROM table_name;
SELECT DISTINCT column_name FROM table_name;