Question

I have a performance issue when selecting data in my project.

There is a table with 3 columns: “id”,”time” and “group”

• The ids are just unique ids as usual.
• The time is the creation date of the entry.
• The group is there to cummulate certain entries together.

So the table data may look like this:

ID | TIME      | GROUP
------------------------
1  | 20090805  | A
2  | 20090804  | A
3  | 20090804  | B
4  | 20090805  | B
5  | 20090803  | A
6  | 20090802  | B

…and so on.

The task is now to select the “current” entries (their ids) in each group for a given date. That is, for each group find the most recent entry for a given date.

Following preconditions apply:

• I do not know the different groups in advance – there may be many different ones changing over time
• The selection date may lie “in between” the dates of the entries in the table. Then I have to find the closest one in each group. That is, TIME is less than the selection date but the maximum of those to which this rule applies in a group.

What I currently do is a multi-step process which I would like to change into single SELECT statement:

1. SELECT DISTINCT group FROM table to find the available groups
2. For each group found in 1), SELECT * FROM table WHERE time<selectionDate AND group=loop ORDER BY time DESC
3. Take the first row of each result found in 2)

Obviously this is not optimal.

So I would be very happy if some more experienced SQL expert could help me to find a solution to put these steps in a single statement.

Thank you!

Answer 1

The following will work on SQL Server 2005+ and Oracle 9i+:

WITH groups AS (
       SELECT t.group,
              MAX(t.time) 'maxtime'
         FROM TABLE t
     GROUP BY t.group)
SELECT t.id,
       t.time,
       t.group
  FROM TABLE t
  JOIN groups g ON g.group = t.group AND g.maxtime = t.time

Any database should support:

SELECT t.id,
       t.time,
       t.group
  FROM TABLE t
  JOIN (SELECT t.group,
               MAX(t.time) 'maxtime'
          FROM TABLE t
      GROUP BY t.group) g ON g.group = t.group AND g.maxtime = t.time