假设我有三个表项目，客户端和工作。像这样
项目表
---------------------------------------------------------
| project_id | project_name | client_id |
---------------------------------------------------------
| 1 | abc | 1 |
| 2 | xyz | 2 |
---------------------------------------------------------
客户表
-------------------------------------
| client_id | client_name |
-------------------------------------
| 1 | a |
| 2 | b |
-------------------------------------
工作表
---------------------------------------------------------
| work_id | work_cost | project_id |
---------------------------------------------------------
| 1 | 100 | 1 |
| 2 | 200 | 1 |
| 3 | 100 | 2 |
| 4 | 100 | 1 |
| 5 | 100 | 1 |
| 6 | 200 | 2 |
---------------------------------------------------------
在这里我要从三个表中查询并显示结果。
------------------------------------------------------------------------
| project_id | project_name | client_name | total_work_cost |
------------------------------------------------------------------------
| 1 | abc | a | 500 |
| 2 | xyz | b | 300 |
------------------------------------------------------------------------
这里total_work_cost表示，假设项目1在工作表中有4条记录。我想对项目1的工作表中的所有work_cost求和。
我已经尝试过这样-
SELECT project.*, c.client_name, SUM(w.work_cost) as total_work_cost
FROM project
LEFT JOIN client as c ON project.client_id = c.client_id
INNER JOIN work as w on project.project_id = w.project_id
ORDER BY projects.project_id
答案 0 :(得分：1)
您缺少聚合逻辑，即查询中的GROUP BY子句：
SELECT p.project_id, p.project_name, c.client_name, SUM(w.work_cost) as total_work_cost
FROM project as p LEFT JOIN client as c
ON p.client_id = c.client_id
LEFT JOIN work as w
on p.project_id = w.project_id
GROUP BY p.project_id, p.project_name, c.client_name
ORDER BY p.project_id