如何在同一选择查询中将一个表中的值交换到另一表中的值？

Question

My database consists of two tables

users table

+---------+----------+---------------+-----------+
| user_id | username | password_hash | user_type |
+---------+----------+---------------+-----------+
| 1       | Admin    | hash          | 0         |
| 2       | Student  | hash          | 1         |
| 3       | Teacher  | hash          | 2         |
+---------+----------+---------------+-----------+

tickets table

+-----------+---------+-----------------+----------------+--------------------+---------------+-------------+
| ticket_id | user_id | ticket_category | ticket_summary | ticket_description | ticket_status | assignee_id |
+-----------+---------+-----------------+----------------+--------------------+---------------+-------------+
| 1         | 2       | Network         | Test Network   | Lorem Ipsum        | 1             | 1           |
| 2         | 3       | Printer         | Test Printer   | Lorem Ipsum        | 1             |             |
+-----------+---------+-----------------+----------------+--------------------+---------------+-------------+

Both tickets.user_id and tickets.assignee_id are foreign keys to users.user_id

I am querying these tables using the following select statement

SELECT username AS `Opened By`, assignee_id AS `Assigned To`, ticket_category AS `Category`, ticket_summary AS `Summary`, ticket_description AS `Description`, IF(ticket_status = 1,'Open','Closed') AS `Status` FROM tickets INNER JOIN users ON tickets.user_id = users.user_id;

This builds a dataset like so

+-----------+-------------+----------+--------------+-------------+--------+
| Opened By | Assigned To | Category | Summary      | Description | Status |
+-----------+-------------+----------+--------------+-------------+--------+
| Student   | 1           | Network  | Test Network | Lorem Ipsum | Open   |
| Teacher   |             | Printer  | Test Printer | Lorem Ipsum | Open   |
+-----------+-------------+----------+--------------+-------------+--------+

In my select statement, I would like to replace the assignee_id number with the username that it corresponds too in the users table.

For example, the first ticket (ticket_id = 1) has an assignee_id of 1 and I would like the Assigned To column to display the username "Admin" instead of 1

How could I modify my select query to accomplish this?

Answer 1

Just join to the users table again for the assignee_id . Note it needs to be a LEFT JOIN in case no one has been assigned to the ticket. You may also want to use COALESCE on u2.username to output a blank instead of (null) (ie COALESCE(u2.username, '') ). SQLFiddle .

SELECT u1.username AS `Opened By`, 
       u2.username AS `Assigned To`, 
       ticket_category AS `Category`, 
       ticket_summary AS `Summary`, 
       ticket_description AS `Description`, 
       IF(ticket_status = 1,'Open','Closed') AS `Status` 
FROM tickets 
JOIN users u1 ON tickets.user_id = u1.user_id
LEFT JOIN users u2 ON tickets.assignee_id = u2.user_id

Output for your sample data:

Opened By   Assigned To   Category   Summary       Description   Status
Student     Admin         Network    Test Network  Lorem Ipsum   Open 
Teacher     (null)        Printer    Test Printer  Lorem Ipsum   Open 

Answer 2

select T.OpenedBy,u.username as AssignedTo,Category,Summary,Description,Status from 
(
 SELECT username AS `OpenedBy`, assignee_id , ticket_category AS `Category`, ticket_summary AS `Summary`, ticket_description AS `Description`, IF(ticket_status = 1,'Open','Closed') AS `Status` FROM tickets INNER JOIN users ON tickets.user_id = users.user_id
) as T

inner join
users u on T.assignee_id=u.user_id