Delete all rows which has no id existing in another table

Question

I want to delete all rows which has no existing foreign key in another table example:

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    |
+----+-------+

The query will delete rows with id# 20 and 40 on table2.

I need to do this so that i could establish a relationship with table1 and table2.

Answer 1

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