Question

I have small trouble creating a query. I have two tables:

user_data
+----+---------+--------+
| id | mail    | etc... |
+----+---------+--------+
|  1 | [e-mail] | ...    |
|  2 | [e-mail] | ...    |
|  3 | [e-mail] | ...    |
+----+---------+--------+

 

contracts
+----+---------+--------+
| id | user_id | etc... |
+----+---------+--------+
|  1 |       1 | ...    |
|  2 |       2 | ...    |
|  3 |       1 | ...    |
|  4 |       1 | ...    |
|  5 |       3 | ...    |
+----+---------+--------+

As you can see, the first table contains data about users and the secound one about their contracts. There will be always only one entry about a user, but a user can have multiple contracts. Now I need to find out
all users, theirs first contract id ( with the lowest id in contracts table ) and their email, if it’s in the were parameters.

So far I have such query:

SELECT
  u.id as user_id,
  c.id as first_contract_id,
  u.mail as email
FROM
  user_data u
JOIN
  contracts c ON u.id = c.user_id
WHERE
  u.mail
IN (
  '[e-mail]',
  '[e-mail]',
  '[e-mail]'
);

Now I have no idea how I can select only the lowest contract ID from these results. Help apreciated.

Answer 1

SELECT
  u.id as user_id,
  min(c.id) as first_contract_id,
  u.Mail as email
FROM
  user_data u
JOIN
  contracts c ON u.id = c.user_id
WHERE
  u.mail IN ('[e-mail]', '[e-mail]', '[e-mail]')
GROUP BY u.id

If you group by the user you can get the lowest contract by using min.

(And MySQL has no problem with selecting column that are not in a group)