mysql - 如何在MySQL列中插入没有匹配的NULL值？

Question

这个问题已经有答案了:

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN? [duplicate] (2 个回答)

已关闭 8 年前。

我有以下两个表:

项目消耗:

+---------------+------------+----------+
| project_name  | project_id | Utilised |
+---------------+------------+----------+
| CStreamTax    |          1 |     3500 |
| CStreamTravel |          2 |      149 |
+---------------+------------+----------+

项目:

+------------+---------------------+
| project_id | project_budgetstart |
+------------+---------------------+
|          1 |               15000 |
|          2 |               15000 |
|          3 |                 500 |
+------------+---------------------+

我想制作一个两者的联合表，但使用 Utilized字段充满 NULL当项目表中有一个project_id时与projects_expished 表中的条目之一不匹配。

我的基本尝试如下:

SELECT projects.*, projects_expended.utilised
  FROM projects_expended
  JOIN projects
    ON projects.project_id = projects_expended.project_id;

结果:

+------------+---------------------+----------+
| project_id | project_budgetstart | utilised |
+------------+---------------------+----------+
|          1 |               15000 |     3500 |
|          2 |               15000 |      149 |
+------------+---------------------+----------+

我明白为什么我会得到这样的结果。但我对此很坚持。我还进行了其他尝试(太多了，无法计数)。

Answer 1

我的答案来自:

LEFT JOIN vs. LEFT OUTER JOIN in SQL Server

这非常有帮助。

解决办法:

SELECT projects.*, projects_expended.utilised
  FROM projects_expended
 RIGHT JOIN projects
    ON projects.project_id = projects_expended.project_id;