SQL的DISTINCT函数用于返回唯一不重复的值。它可以用于SELECT语句的列名之前，也可以用于COUNT、SUM、AVG等聚合函数之前。

以下是DISTINCT函数的用法示例：

SELECT DISTINCT column_name FROM table_name;

SELECT COUNT(DISTINCT column_name) FROM table_name;

SELECT column_name1, column_name2, … FROM table_name WHERE column_name1 = value1 AND column_name2 = value2;

SELECT DISTINCT column_name1, column_name2 FROM table_name ORDER BY column_name1 ASC, column_name2 DESC;

需要注意的是，DISTINCT函数只能用于查询列，而不能用于查询表达式或函数。