Question

我是SQL的新手，需要返回从2018年或更晚开始的每个办公室的员工人数。对于没有任何员工在01/01/2018之后开始的办公室，结果应为零。

Employees
---------------------------------
|id|office_id|name    |start_date|
---------------------------------|
|1 | 1       | Kyle   |2018-08-08|
|2 | 1       | Sarah  |2017-02-28|
|3 | 3       | Moe    |2019-01-01|
|4 | 3       | Adi    |2018-11-04|
|5 | 3       | Kwun   |2016-05-22|
---------------------------------


Offices
---------
|id|name |
---------|
|1 | CHI |
|2 | NYC |
|3 | LA  |
---------

这将返回正确的计数，但是我需要它为办公室ID＃2（NYC）返回零。我想念什么？

SELECT o.id, COUNT(e.id) AS NumberOfEmployees
FROM Offices AS o
LEFT JOIN Employees as e 
  ON office_id = o.id
WHERE start_date >= '2018-01-01'
GROUP BY o.id

Answer 1

您必须将start_date >= '2018-01-01'条件移到左联接中。这样，就可以看到没有员工的办公室：

SELECT o.id, COUNT(e.id) AS NumberOfEmployees
FROM Offices AS o
LEFT JOIN Employees as e 
  ON office_id = o.id 
      AND start_date >= '2018-01-01'
GROUP BY o.id

计数后，SQL查询不会返回零

1 个答案: