删除另一个表中没有id的所有行

Question

我想删除另一个表中没有现有外键的所有行 例如：

table1
+----+-------+
|id  | data  |
+----+-------+
| 1  | hi    |
+----+-------+
| 2  | hi    |
+----+-------+
| 3  | hi    |
+----+-------+
| 4  | hi    |
+----+-------+
| 5  | hi    |
+----+-------+

table2
+----+-------+
|a_id| data  |
+----+-------+
| 1  | hi    |
+----+-------+
| 20 | hi    |
+----+-------+
| 3  | hi    |
+----+-------+
| 40 | hi    |
+----+-------+
| 5  | hi    |
+----+-------+

查询将删除table2上ID为＃20和40的行。

我需要这样做才能与table1和table2建立关系。

Answer 1

DELETE table2 
FROM   table2 
       LEFT JOIN table1 
              ON table2.a_id = table1.id 
WHERE  table1.id IS NULL 

Answer 2

总之，有一些树方法可以删除多个表

1. NOT IN(SELECT ...) - @someone（他已删除了答案）

   Delete From Tab2 where ID not in (Select ID From Tab1)
   

2. LEFT JOIN - @eggyal

   DELETE table2
   FROM   table2 LEFT JOIN table1 ON table2.a_id = table1.id
   WHERE  table1.id IS NULL
   

3. NOT EXISTS

   DELETE
   FROM  table2
   WHERE NOT EXISTS (
     SELECT 1
     FROM table1
     WHERE table1.id = table2.a_id
   )
   

根据What's the difference between NOT EXISTS vs. NOT IN vs. LEFT JOIN WHERE IS NULL?，不同的RDBMS表现不同。