两列组合唯一的SQL查询（带有子查询）

Question

I have two tables; one with contact info, and one with a list of groups. Eg:

clients
+-------------+---------------+-------------+------------+
|  client_id  |  client_name  |  group1_id  |  group2_id |
+-------------+---------------+-------------+------------+
| 283         | John Smith    | 1           | null       |
| 284         | Jane Smith    | 1           | 2          |
| 285         | Tim Johnson   | 1           | null       |
| 286         | Peter Guy     | null        | null       |
+-------------+---------------+-------------+------------+

groups
+------------+----------------+
|  group_id  |  group_name    |
+------------+----------------+
|  1         |  Smith Group   |
|  2         |  Johnson Group |
+------------+----------------+

In case it's not self-evident, group1_id and group2_id are foreign keys into the groups table.

I need to write a query to select the client name and group name. If a client is a member of two groups (like Jane Smith) they should appear twice in the output. Ideally each line should output the group name, followed by the client name in parenthesis, and all results should be sorted alphabetically. So for the above tables the output should be:

John Smith (Jane Smith)
Johnson Group (Jane Smith)
Johnson Group (Tim Johnson)
Smith Group (John Smith)

Is this possible using a single SQL query?

I'm using MySQL. I'm in the planning stage of the project at the moment so if I need to change the database schema to better support doing I can.

Thanks in advance.

Answer 1

Yes this is possible using a join with in or or .

select c.client_name, g.group_name
from clients c 
   join groups g on g.group_id in (c.group1_id, c.group2_id)

• SQL Fiddle Demo

---

However, I would suggest looking into database normalization . Altering your table structure to include a 3rd table called ClientGroup would be better to work as a junction table. Then you would end up with something like this:

clients (client_id, client_name)
groups (group_id, group_name)
clientgroup (client_id, group_id)

And then you could just join the tables again together:

select c.client_name, g.group_name
from clients c
    join clientgroup cg on c.client_id = cg.client_id
    join groups g on cg.group_id = c.group_id

• A Visual Explanation of SQL Joins

Answer 2

If you want two rows when a client matches twice, then use or :

select c.client_name, g.group_name
from clients c join
     groups g
     on g.group_id = c.group1_id or g.group_id = c.group2_id;

This formats the results as two columns, rather than a string that puts the group name in parentheses.

Answer 3

Thank you sgeddes and Gordon for your quick and useful answers. I figured I'd need a join but was having trouble getting the syntax right (I've not used joins before). The end result was quite obvious after seeing someone else give the answer!

With the sorting and client name in parenthesis, the final query is:

SELECT client_id, CONCAT(g.group_name, " (", c.client_name, ")") new_col
FROM clients c 
    JOIN groups g on g.group_id IN (c.group1_id, c.group2_id)
ORDER BY g.group_id, c.client_name ASC

I'm a first-time poster and can't see a way to award an answer. Am I missing something, or is it only questions that offer a bounty that allow awarding?