SQL query with grouping and MAX - sql

Question

I have a table that looks like the following but also has more columns that are not needed for this instance.
ID DATE Random
-- -------- ---------
1 4/12/2015 2
2 4/15/2015 2
3 3/12/2015 2
4 9/16/2015 3
5 1/12/2015 3
6 2/12/2015 3
ID is the primary key
Random is a foreign key but i am not actually using table it points to.
I am trying to design a query that groups the results by Random and Date and select the MAX Date within the grouping then gives me the associated ID.
IF i do the following query
select top 100 ID, Random, MAX(Date) from DateBase group by Random, Date, ID
I get duplicate Randoms since ID is the primary key and will always be unique.
The results i need would look something like this
ID DATE Random
-- -------- ---------
2 4/15/2015 2
4 9/16/2015 3
Also another question is there could be times where there are many of the same date. What will MAX do in that case?

Answer 1

You can use NOT EXISTS() :
SELECT * FROM YourTable t
WHERE NOT EXISTS(SELECT 1 FROM YourTable s
WHERE s.random = t.random
AND s.date > t.date)
This will select only those who doesn't have a bigger date for corresponding random value.
Can also be done using IN() :
SELECT * FROM YourTable t
WHERE (t.random,t.date) in (SELECT s.random,max(s.date)
FROM YourTable s
GROUP BY s.random)
Or with a join:
SELECT t.* FROM YourTable t
INNER JOIN (SELECT s.random,max(s.date) as max_date
FROM YourTable s
GROUP BY s.random) tt
ON(t.date = tt.max_date and s.random = t.random)

Answer 2

In SQL Server you could do something like the following,
select a.* from DateBase a inner join
(select Random,
MAX(dt) as dt from DateBase group by Random) as x
on a.dt =x.dt and a.random = x.random

Answer 3

This method will work in all versions of SQL as there are no vendor specifics (you'll need to format the dates using your vendor specific syntax)
You can do this in two stages:
The first step is to work out the max date for each random:
SELECT MAX(DateField) AS MaxDateField, Random
FROM Example
GROUP BY Random
Now you can join back onto your table to get the max ID for each combination:
SELECT MAX(e.ID) AS ID
,e.DateField AS DateField
,e.Random
FROM Example AS e
INNER JOIN (
SELECT MAX(DateField) AS MaxDateField, Random
FROM Example
GROUP BY Random
) data
ON data.MaxDateField = e.DateField
AND data.Random = e.Random
GROUP BY DateField, Random
SQL Fiddle example here: SQL Fiddle
To answer your second question:
If there are multiples of the same date, the MAX(e.ID) will simply choose the highest number. If you want the lowest, you can use MIN(e.ID) instead.