统计客户的实例答案

【问题标题】：Counting the instances of customers统计客户的实例
【发布时间】：2010-06-15 15:57:37
【问题描述】：

假设我有一个表，其中有一列名为 CustomerId。该表的实例示例为：

CustomerId
14
12
11
204
14
204

我想编写一个计算客户 ID 出现次数的查询。最后，我希望得到这样的结果：

CustomerId      NumberOfOccurences
14              2
12              1
11              1
204             2
14              1

我想不出办法来做到这一点。

【问题讨论】：

这听起来很像家庭作业...如果是，请标记它。
不，不是。我现在正在做一个项目。

标签： sql group-by

【解决方案1】：

这是 GROUP BY 最基本的例子

SELECT CustomerId, count(*) as NumberOfOccurences
    FROM tablex GROUP BY CustomerId;

【讨论】：

【解决方案2】：

此页面上的Practice exercise #3 解释了如何执行此操作。

CREATE TABLE customers
(   customer_id     number(10)  not null,
    customer_name   varchar2(50)    not null,
    city    varchar2(50),   
    CONSTRAINT customers_pk PRIMARY KEY (customer_id)
);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7001, 'Microsoft', 'New York');

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7002, 'IBM', 'Chicago');

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7003, 'Red Hat', 'Detroit');

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7004, 'Red Hat', 'New York');

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7005, 'Red Hat', 'San Francisco');

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7006, 'NVIDIA', 'New York');

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7007, 'NVIDIA', 'LA');

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7008, 'NVIDIA', 'LA');

解决方案：

以下 SQL 语句将返回 customers 表中每个 customer_name 的不同城市数：

SELECT customer_name, COUNT(DISTINCT city) as "Distinct Cities"
FROM customers
GROUP BY customer_name;

它将返回以下结果集：

CUSTOMER_NAME   Distinct Cities
IBM     1
Microsoft   1
NVIDIA  2
Red Hat     3

【讨论】：