find the max for each value in SQL - sql

Question

i have tables like this
table 1
|cl.1|
| -- |
| a |
| b |
| c |
table 2
|cl.1|cl.2|para|
|----|---| --- |
| a | 3 | t |
| a | 3 | f |
| b | 2 | t |
| a | 1 | b |
| c | 4 | t |
| b | 7 | d |
i want to get the max value for each element in table1 from table2
and the different parameter
so the expecited tabel should be like this
|cl.1|max|para|
|----|---| --- |
| a | 3 | t |
| a | 3 | f |
| c | 4 | t |
| b | 7 | d |

Answer 1

You can try to compute all the maximums:
with Maxes as (
select cl1,
max(cl2) as cl2
from Table2
group by cl1)
and then join them with the original Table2, e.g.
with Maxes as (
select cl1,
max(cl2) as cl2
from Table2
group by cl1)
select t.*
from Table2 t join
Maxes m on (t.cl1 = m.cl1 and t.cl2 = m.cl2)

Answer 2

Depends on what features your RDBMS supports.
With Oracle you could do a CROSS APPLY to order table2 by descending cl2 and keep the top values (with ties):
select T1.c1, TM.maximum, TM.para
from Table1 T1
cross apply (
select *
from Table2 T2
where T2.c1 = T1.c1
order by T2.maximum descending
fetch first 1 row with ties
) TM
You can do the same in SQL Server with syntax select top 1 with ties instead of fetch first 1 row with ties.
Another option could be to use Analytical Functions to rank the results per col1 and then keep only the first ones.
select T.c1, T.maximum, T.para
from (
select
T1.c1, T2.maximum, T2.para,
rank() over (partition by T1.c1 order by T2.maximum desc) r
from T1
join T2 on T1.c1 = T2.c2
) T
where T.r = 1
Less stylish and probably(?) less performant would be computing the maximum for each c1 and then doing an equality:
select T1.c1, T2.maximum, T2.para
from T1
join T2 on T1.c1 = T2.c1
where T2.maximum = (select max(maximum) from T2 where c1 = T1.c1)

Answer 3

If you are trying to get the max tl.1 and if for the same values it is equal, you could try:
SELECT *
FROM table2
WHERE cl_2 in ( SELECT MAX(cl_2)
FROM table2
group by cl_1
);
Result:
cl_1 cl_2 para
a 3 t
a 3 f
c 4 t
b 7 d
Tested on MySQL : https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=42a6bc20622a210b18101588540995ec
You could use a join , but it makes no difference:
SELECT t1.cl_1,t2.cl_2,t2.para
FROM table2 t2
INNER JOIN table1 t1 on t2.cl_1=t1.cl_1
WHERE t2.cl_2 in (SELECT MAX(cl_2) FROM table2 group by cl_1 );
Demo: https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=4b2eed9bcee3532cc7c4e7b3862bc3ef

Answer 4

DENSE_RANK can be used to get whole rows that have a maximum of something within a partition.
Because when sorted descending, the top 1 will have rank 1.
select cl_1, cl_2, para
from
(
select cl_1, cl_2, para
, dense_rank() over (partition by cl_1 order by cl_2 desc) as rnk
from table1 t1
join table2 t2 using (cl_1)
) q
where rnk = 1

Answer 5

Use a CTE to get the max values, then select the rows with those values:
with maxes as
(
select t1.[cl.1]
, max(t2.[cl.2]) max_val
from table1 t1
inner join table2 t2
on t1.[cl.1] = t2.[cl.1]
group by t1.[cl.1]
)
select t1.[cl.1]
, t2.[cl.2]
, t2.para
from table1 t1
inner join table2 t2
on t1.[cl.1] = t2.[cl.1]
where t2.[cl.2] = (select m.max_val from maxes m where m.[cl.1] = t1.[cl.1])
This can also be achieved by joining the CTE:
with maxes as
(
select t1.[cl.1]
, max(t2.[cl.2]) max_val
from table1 t1
inner join table2 t2
on t1.[cl.1] = t2.[cl.1]
group by t1.[cl.1]
)
select t1.[cl.1]
, t2.[cl.2]
, t2.para
from table1 t1
inner join table2 t2
on t1.[cl.1] = t2.[cl.1]
inner join maxes m
on t2.[cl.2] = m.max_val