NULL 連接表的值缺失

Question

這是一個簡單的例子： sqlfiddle.com /#!9/8087c5/1

我有以下查詢：

select
    contracts.ID as contractID,
    departments.ID as departmentID,
    max(invoices.timestamp) as timestamp
from contracts
    left join departments on departments.ID = contracts.departmentID and departments.accountingType > 0
    left join invoices on contracts.ID = invoices.contractID
group by contractID
having departmentID is not null;

我期望得到如下結果：

+------------+--------------+---------------------+
| contractID | departmentID | timestamp           |
+------------+--------------+---------------------+
| 101        | 301          | NULL                |
+------------+--------------+---------------------+
| 102        | 302          | NULL                |
+------------+--------------+---------------------+
| 103        | 303          | 2020-05-01 11:11:00 |
+------------+--------------+---------------------+

相反，我得到：

+------------+--------------+---------------------+
| contractID | departmentID | timestamp           |
+------------+--------------+---------------------+
| 103        | 303          | 2020-05-01 11:11:00 |
+------------+--------------+---------------------+

我不明白為什么查詢會削減最后一個左連接表的 NULL 值。 我錯過了一些非常簡單的東西嗎？

Answer 1

好的 - 問題是group by contractID正確應該是group by contracts.ID 。 按連接發票 talbes 列 contractID 分組的group by contractID分組，這就是缺少列的原因。