Question

表：客户

customer_id  cust_name     city        grade       salesman_id
-----------  ------------  ----------  ----------  -----------
3002         Nick Rimando  New York    100         5001
3005         Graham Zusi   California  200         5002
3001         Brad Guzan    London                  5005
3004         Fabian Johns  Paris       300         5006
3007         Brad Davis    New York    200         5001
3009         Geoff Camero  Berlin      100         5003
3008         Julian Green  London      300         5002
3003         Jozy Altidor  Moncow      200         5007

表：订单

ord_no      purch_amt   ord_date    customer_id  salesman_id
----------  ----------  ----------  -----------  -----------
70001       150.5       2012-10-05  3005         5002
70009       270.65      2012-09-10  3001         5005
70002       65.26       2012-10-05  3002         5001
70004       110.5       2012-08-17  3009         5003
70007       948.5       2012-09-10  3005         5002
70005       2400.6      2012-07-27  3007         5001
70008       5760        2012-09-10  3002         5001
70010       1983.43     2012-10-10  3004         5006
70003       2480.4      2012-10-10  3009         5003
70012       250.45      2012-06-27  3008         5002
70011       75.29       2012-08-17  3003         5007
70013       3045.6      2012-04-25  3002         5001

我需要一个查询，生成每个客户的名称和编号，并且有多个当前订单。

我的尝试： -

SELECT customer_id,  cust_name
FROM customer a
WHERE (SELECT COUNT (*)
       FROM orders b
       WHERE a.customer_id = b.customer_id) > 1

这提供了所需的输出。但是，是否有任何替代方法可以提供所需的输出并且比这更好？另外，我怎么知道哪一个会做得更好？

Answer 1

我试试这个：

SELECT customer_id,  cust_name
FROM customer a
WHERE customer_id IN (SELECT customer_id
                      FROM orders b
                      GROUP BY customer_id
                      HAVING COUNT(*) > 1)

子查询只返回在orders表中存在一次的customer_id。

或者这与上面类似，但JOIN代替IN：

SELECT a.customer_id, a.cust_name
FROM customer a
JOIN (SELECT customer_id
      FROM orders b
      GROUP BY customer_id
      HAVING COUNT(*) > 1) b ON a.customer_id = b.customer_id

Answer 2

您可以让查询过滤具有多个订单的客户作为内部查询，并加入其中：

SELECT c.customer_id, c.cust_name
FROM   customer c
JOIN   (SELECT   customer_id
        FROM     orders
        GROUP BY customer_id
        HAVING   COUNT(*) > 1) o ON c.customer_id = o.customer_id

Answer 3

没有真正需要带有子查询的IN 这可以通过连接完成。

SELECT o.customer_id, c.cust_name, count(*) as total
FROM orders o 
LEFT JOIN customer c ON (o.customer_id = c.customer_id)
GROUP BY o.customer_id, c.cust_name
HAVING count(*) > 1;

至于比较哪个查询可能表现更好？查看有关获取查询执行计划的this old post。

但是如果你需要“当前”订单，你可能想要添加一个where子句来只选择最近日期的订单。

Answer 4

使用join和Group by：

SELECT customer_id,  cust_name
FROM customer a join order a.customer_id = b.customer_id
Group by customer_id,  cust_name

join仅返回在订单表中有记录的客户。如果有多个订单，Group by会阻止您获取多个值。

还有其他方法可以解决这个问题吗？

4 个答案: