Selecting values from second column alongside the values from first column in the same row

I am trying to get values matching the value from the second column. For example, I want to know who is the sender for Bill Gates by only using IDs.

I have two tables,

*users* table
| user_ID  | Full_name      |
| -------- | -------------- |
| 1        | Steve Jobs     |
| 2        | Bill Gates     |
| 3        | Elon Musk      |

*relationships* table (with both column foreign keys)
| user_sender  | user_receiver  |
| ------------ | -------------- |
| 1            | 2              |
| 3            | 1              |
| 3            | 2              |

I want to select based on “user_receiver” column the matching values in the column “user_sender”

For example, I want to know who is user_sender for 2 OUTPUT:

|              |                |
| ------------ | -------------- |
| 1            | 2              |
| 3            | 2              |

Answer

You need to join the tables and select the rows you want

you have access to all columns of both tables by addressing them with their alias

SELECT u.user_ID  , u.Full_name,r.user_receiver 
 FROM users u JOIN 
relationships r ON u.user_ID = r.user_sender
WHERE r.user_receiver = 2