Max rows by group - sql

Question

Current SQL:
select t1.*
from table t1
where t1.id in ('2', '3', '4')
Current results:
id | seq
---+----
3 | 5
2 | 7
2 | 5
3 | 7
4 | 3
Attempt to select maxes:
select t1.*
from table t1
where t1.id in ('2', '3', '4')
and t1.seq = (select max(t2.seq)
from table2 t2
where t2.id = t1.id)
This obviously does not work since I'm using an in list. How can I adjust my SQL to get these expected results:
id | seq
---+----
2 | 7
3 | 7
4 | 3

Answer 1

Group By is your friend:
SELECT
id,
MAX(seq) seq
FROM TABLE
GROUP BY id
EDIT: Response to comment. To get the rest of the data from the table matching the max seq and id just join back to the table:
SELECT t1.*
FROM TABLE t1
INNER JOIN (
SELECT
id
MAX(seq) as seq
FROM TABLE
GROUP BY id
) as t2
on t1.id = t2.id
and t1.seq = t2.seq
EDIT: Gordon and Jean-Francois are correct you can also use the ROW_NUMBER() analytic function to get the same result. You need to check the performance difference for your application (I did not check). Here is an example of that:
SELECT *
FROM (
SELECT ROW_NUMBER() OVER (
PARTITION BY id
ORDER BY seq DESC) as row_num
,*
FROM TABLE
) as TMP
WHERE row_num = 1

Answer 2

This SQL Query will give you max seq from individaul ID.
SELECT t1.*
FROM t1
WHERE t1.id in ('2', '3', '4')
AND NOT EXISTS (
SELECT *
FROM t1 t2
WHERE t2.id = t1.id
AND t2.seq > t1.seq

Answer 3

select *
from table
where (id,seq) in
(
select id,max(seq)
from table
group by id
having id in ('2','3','4')
);
That is if id and/or seq are completely part of the PK of that table.

Answer 4

Here's another example, using the first/last method I mentioned earlier in the comments:
with sd as (select 3 id, 5 seq, 1 dummy from dual union all
select 2 id, 7 seq, 2 dummy from dual union all
select 2 id, 5 seq, 3 dummy from dual union all
select 3 id, 7 seq, 4 dummy from dual union all
select 3 id, 7 seq, 5 dummy from dual union all
select 4 id, 3 seq, 6 dummy from dual)
select id,
max(seq) max_seq,
max(dummy) keep (dense_rank first order by seq desc) max_rows_dummy
from sd
group by id;
ID MAX_SEQ MAX_ROWS_DUMMY
---------- ---------- --------------
2 7 2
3 7 5
4 3 6
The keep (dense_rank first order by ...) bit is requesting to keep the values associated with the rank of 1 in the order list of rows. The max(...) bit is there in case more then one row has a rank of 1; it's just a way of breaking ties.