Question

我有两个表

表1

+-----------+----------+
| support_id| Name     |
+-----------+----------+
| 1         | Name 1   |
| 2         | Name 2   |
+-----------+----------+

表2

+-----------+----------+
| school_id | support_id|
+-----------+----------+
| 2314      | 1        |
+-----------+----------+

期望输出

+-----------+----------+------------+
| school_id |support_id|     has    |
+-----------+----------+------------+
| 2314      | 1        | Yes        |
| 2314      | 2        | No         |
+-----------+----------+------------+

如何添加第三行告诉我表2是否在表1中？

谢谢！

Answer 1

表1在表2中是什么意思？字段support_id与表2中的support_id相同吗？还是字段或school_id相同？

两个表中的字段必须相同，以便建立连接（外键），进行联接并连接两个表。

考虑将support_id作为Foreigh键（两个表之间共享的值），您可以使用以下选择：

   SELECT school_id, IF(support_id is not null, "Yes", "No") as Has
   FROM table1 LEFT JOIN table2
   ON table1.support_id = table2.support_id;

Answer 2

我将使用左联接，然后使用case表达式来格式化结果：

SELECT    t2.*, CASE WHEN t1.support_id IS NOT NULL THEN 'Yes' ELSE 'No' END
FROM      t2
LEFT JOIN t1 ON t2.support_id = t1.support_id

连接2个表并创建一个新行，显示表1中是否存在表2

2 个答案: