sql - SQL：只打印一次表中的名称

Question

我正在尝试列出下订单的客户姓名，他们来自某个县并从某个销售代表下订单。问题是，决赛桌会多次打印出他们的名字。我如何让他们每个人只打印一次名字？

SELECT cName
FROM customer INNER JOIN orders ON customer.cNo = orders.cNO,author
WHERE county = "Galway" AND saleRepName = "Jane Smith"; 

Answer 1

您可以使用Distinct，但如果有很多记录，您可能会受到性能影响

SELECT DISTINCT
    cName 
FROM customer 
    INNER JOIN orders ON customer.cNo = orders.cNO,author 
WHERE county = "Galway" AND saleRepName = "Jane Smith";

Group By可能会更快，并允许您获得计数的额外好处：

SELECT
    count(1),
    cName 
FROM customer 
    INNER JOIN orders ON customer.cNo = orders.cNO,author 
WHERE county = "Galway" AND saleRepName = "Jane Smith";
GROUP BY cName

这将打印出如下内容：

3 John Doe
6 Jane Smith
1 Jersey Mike