Question

我有下表

dbo.employee

id          name      position_id      phone
-----------------------------------------------   
1           Gosho         1            5623565
2           Pesho         2            095656
3           Misho         3            0984654

dbo.repairs

id          service_id  client_id   employee_id    dt      price
----------- ----------- ----------- ----------- ---------- ------
3           1           2           1           2003-05-03 4.54
4           2           1           2           2013-05-03 3.55
5           1           2           1           2003-05-03 4.54
6           2           1           2           2013-05-03 3.55
7           1           2           1           2003-05-03 4.54
8           2           1           2           2013-05-03 3.55
9           1           2           1           2003-05-03 4.54
10          2           1           2           2013-05-03 3.55

我想查看每位员工有多少次维修。

我想要的结果应该是这样的：

employee_id  repairs_count
--------------------------
1             4
2             4

这是我想到的代码

SELECT 
    COUNT(r.employee_id)
FROM
    repairs r
LEFT JOIN 
    repairs e ON r.id = e.id
WHERE 
    r.id = e.id

谢谢！

Answer 1

按employee_id分组。像count()这样的汇总函数适用于每个组

select employee_id, count(*) as repairs_count
from repairs
group by employee_id

Answer 2

如果您只关心需要维修的员工（如样本结果中所示），则不需要join：

select r.employee_id, count(*) as cnt
from dbo.repairs r
group by r.employee_id;

如果您要所有名员工，即使没有维修的员工，也可以使用left join：

select e.id, count(r.employee_id) as cnt
from dbo.employees e left join
     dbo.repairs r
     on e.employee_id = r.employee_id
group by e.id;

Answer 3

使用JOIN从员工表中添加更多信息

SELECT 
   employee_id = e.id,
   e.name, 
   repairs_count = COUNT(r.id)
FROM
   dbo.employee e
   LEFT JOIN
   dbo.repairs r ON e.id = r.employee_id
GROUP BY
   e.id, e.name
HAVING
   COUNT(r.id) > 0

Answer 4

有几种解决方法

分组依据

select e.id,
       e.name,
       e.position_id,
       e.phone,
       count(*) as repairs_count
from   employee e
  left outer join repairs r on e.id = r.employeeid
group by e.id, e.name, e.position_id, e.phone

注意：如果您要舍弃没有维修的员工，只需用内部联接替换左联接

或通过子查询

select e.id,
       e.name,
       e.position_id,
       e.phone,
       (select count(1) from repairs r where r.employeeid = e.id) as repairs_count
from   employee e

我会选择第一个选项，因为优化器将更易于使用索引，并且如果需要的话，它更容易遗漏无法修复的员工

获取SQL Server中每个ID的计数

4 个答案: