Question

I have a problem with getting some ordering of records and uniqueness. I have the following setup of tables:

person
groups
group_has_person

The idea here is to sort persons into groups. (think of Google+ circles)

First I had only a person table with 2 ID fields linking to the group records but then I was always limited to having a person only in 2 groups. Now I have added a linking table so the person can be placed into an unlimited amount of groups. Every group has a sort index field (group.order_index). This is an integer defining the sort-order of the groups. The idea is to show all persons grouped by group and showing every person only once but also order not by name or id but but by the extra index field of the group. So actually I want to show all persons even when they’re not in a group. But when they are in one or more groups I want to show them in the order of the group.order_index and only once.

The query result should look like this:

person.name   person.person_id   group.group_id   group.order_index 
Rick          1                  1                1
Tom           2                  1                1
Jan           4                  3                2
Kees          3                  3                2
Piet          5                  NULL             NULL

Notice that the group_id is not the column to order by but that table has an extra field so the order can be changed after creation of a group.

I got some queries that got close to my expected result but I’m not there yet:

This following query gives the expected sort result but it still gives a person multiple times when it is in multiple groups:

SELECT
  person.person_id AS 'person.person_id',
  person.name AS 'person.name',
  group.order_index AS 'group.order_index',
FROM
  `person`
LEFT OUTER JOIN `group_has_person` ON person.person_id = group_has_person.person_id
LEFT OUTER JOIN `group` ON group_has_person.group_id = group.group_id
ORDER BY
  3 ASC , 
  2 ASC

The key-word DISTINCT didn’t help either.

The following query gives unique persons but the order by group_order_index doesn’t work and even shows faulty numbers:

SELECT
  person.person_id AS 'person.person_id',
  person.name AS 'person.name',
  group.order_index AS 'group.order_index',
FROM
  `person`
LEFT OUTER JOIN `group_has_person` ON person.person_id = group_has_person.person_id
LEFT OUTER JOIN `group` ON group_has_person.group_id = group.workgroup_id
GROUP BY
  group_has_person.person_id
ORDER BY
  3 ASC , 
  2 ASC

Also grouping by person.person_id doesn’t help getting the right order.

Inner joins also don’t work because they wont show persons that are not in a group. Is it possible to get what I want without code and only by a query without having sub-selects?

Answer 1

You are almost there, but you need an aggregate function on order_index:

SELECT
person.person_id AS 'person.person_id',
person.name AS 'person.name',
MIN(group.order_index) AS 'group.order_index',
FROM person
LEFT OUTER JOIN group_has_person ON person.person_id = group_has_person.person_id
LEFT OUTER JOIN group ON group_has_person.group_id = group.workgroup_id
GROUP BY
person.person_id, person.name
ORDER BY
3 ASC, 2 ASC

In other words, if a person belongs to several groups, you can’t avoid duplicates unless you pick which order_index to sort by using min, max, avg or sum.