Question

I have a table with the fields id, group, left, level and createdAt.

Every row belongs to a group. The row with level = 0 is the group “leader”.

I want to sort the table by the leaders’ date, and within each group sort the rows by left. For example, in this table:

Id - Group - Left - Level - CreatedAt
1    1       1      0       00:10
2    1       2      1       00:20
3    2       1      0       00:00
4    1       3      1       00:30
5    2       2      1       00:40

The order should be:

Id - Group - Left - Level - CreatedAt
3    2       1      0       00:00
5    2       2      1       00:40
1    1       1      0       00:10
2    1       2      1       00:20
4    1       3      1       00:30

Because row 3 is the newest group leader, it should be first and followed by all it’s group ordered by left. After that is row 1 which is the second most new leader, followed by it’s group ordered by left.
Etc..

I hope I explained it clear enough.

Thanks!

Answer 1

Essentially, you need to join your table with the leader’s time:

SELECT my_table.*
FROM   my_table NATURAL JOIN (
  SELECT   my_table.Group, MIN(my_table.CreatedAt) AS LeaderTime
  FROM     my_table
  WHERE    my_table.Level = 0
  GROUP BY my_table.Group
) t
ORDER BY t.LeaderTime, my_table.Left

See it on sqlfiddle.

If you can guarantee that there is an unambiguous leader for every group—e.g. because you have defined a UNIQUE constraint on (Group, Level), which you cannot have because your example contains two records in Group = 1 with Level = 1—then you can avoid the grouping operation:

SELECT   my_table.*
FROM     my_table JOIN my_table AS leader
      ON leader.Group = my_table.Group AND leader.Level = 0
ORDER BY leader.CreatedAt, my_table.Left