MySQL 连接查询构造

Question

我有两个表:工作和事件。 一项工作由零个或多个事件组成。

JOBS (id,client_name,job_cost):

1, 'Kelly, Peter', '139.00'


ACTIVITIES (id,job_id,activity_name,activity_cost):

1, 1, 'Activity A', '40.00'

2, 1, 'Activity B', '59.00'

3, 1, 'Activity C', '40.00'

如何编写查询来检索以下字段:

DESIRED OUTPUT FIELDS:

job_id, client_name, job_cost, total_cost_of_activities, number_of_activities

感谢任何帮助，因为 MySQL 不是我的强项之一!

Answer 1

SELECT j.id, j.client_name, j.job_cost, 
COUNT(a.id) AS number_of_activities, SUM(activity_cost) as total_cost_of_activities
FROM Jobs AS j
INNER JOIN Activities AS a ON j.id = a.job_id
GROUP BY j.id, j.client_name, j.job_cost

请注意:数字类型不应存储为 VARCHAR(您将它们写在引号中)。为了提高速度和节省空间，您应该使用数字数据类型，例如 DECIMAL。