group by 中重复使用distinct

使用Group by语句可以对数据进行分组操作，并根据指定的列对数据进行聚合计算。在实际应用中，有时候需要在Group by语句中使用distinct关键字来去除重复的数据，以便更精确地统计和分析数据。

sql
SELECT 
    customer_name, 
    COUNT(DISTINCT order_number) AS order_count, 
    SUM(order_amount) AS total_amount
FROM 
    orders
GROUP BY 
    customer_name;

sql
SELECT 
    student_id, 
    COUNT(DISTINCT course_id) AS course_count
FROM 
    students
GROUP BY 
    student_id;