Simple SQL Questions with join - sql

Question

i have some problems with join tables:
Table A -> ID,Col1,Col2,Col3
Table B -> Rank , ColX , A_ID (Relationship with A.ID)
I want to take higher Rank (each A_ID , like group by A_ID) of B table
my results must be something like A.ID , Col1 , Col2 , Col3 , ""ColX"" , how can i do that ?
and i want my result count equals to A.ID count.
TableA
+--------------------+
| ID|Col1|Col2|Col3| |
+--------------------+
| 1 | C1 | C2 | C3 |
| 2 | C1 | C2 | C3 |
+--------------------+
TABLE_B
+-----------------------------+
| ID| COL_X |RANK |A_ID| |
+-----------------------------+
| 1 | SomeValue | 1 | 1 |
| 2 | some22222 | 2 | 1 |
| 3 | SOMEXXXX | 3 | 1 |
| 4 | SOMEVAL | 1 | 2 |
| 5 | VALUE | 2 | 2 |
+-----------------------------+
Expected Output:
+--------------------------------------------------------------------+
| ID| Col1| Col2 | Col3| COLX |
+--------------------------------------------------------------------+
| 1 | C1 | C2 | C3 | SOMEXXXX (Higher Rank of TableB-> A_ID = 1) |
| 2 | C1 | C2 | C3 | VALUE (Higher Rank of TableB-> A_ID = 2) |
+--------------------------------------------------------------------+

Answer 1

You could easily do this using a subquery by first finding the max for each A_ID and then joining to tableA and TableB to get your desired rows:
SELECT a.ID,
a.col1,
a.Col2,
a.Col3,
b1.Col_X
FROM (
SELECT a_id
,max(rank) AS MaxRank
FROM tableb
GROUP BY a_id
) b
INNER JOIN tablea a ON a.id = b.a_id
INNER JOIN tableb b1 ON b.a_id = b1.a_id AND b1.rank = b.MaxRank
ORDER BY a.ID;
SQL Fiddle Demo

Answer 2

I'm thinking you want to take the max rank from your table b for each row in table a?
There's lots of different ways of approaching this. Here's one simple one:
with maxCTE as
(select
a_id,
max(rank) as MaxRank
from
tableb
group by
a_id
)
select
*
from
tablea a
inner join tableb b
on a.id = b.a_id
inner join maxcte c
on b.a_id = c.a_id
and b.rank = c.MaxRank
SQLFiddle
Basically, the CTE identifies the max rank for each a_id, then we join that back to tableb to get the details about that row.

Answer 3

with x as
(select a_id, max(rank) as mxrnk
from tableB
group by a_id)
select a.id, a.col1, a.col2, a.col3, b.col_x
from tableA a join x
on a.id = x.a_id
join tableB b
on x.mxrnk = b.rank
You can select max rank per a_id in the cte and use it to select the corresponding columns.

Answer 4

One is to INNER JOIN Table B onto Table A by ID's. You will have 3 records returned from Table B. If you ORDER those records by the COLX
SELECT
,a.ID
,a.Col1
,a.Col2
,a.Col3
,b.COLX
FROM TableA AS a
INNER JOIN TABLE_B AS b on b.A_ID = a.id
ORDER BY b.COLX DESC
Then another way is joining a sub query of Table B that also has a sub query that filters Table B records to only the records with the highest RANK.
That way you can bring in COLX values from the highest RANK records from Table B that match the records of Table A.
I think at least...
SELECT
a.ID
,a.Col1
,a.Col2
,a.Col3
,b.COLX
FROM TableA a
INNER JOIN (
SELECT
a.A_ID
,a.RANK
,a.COLX
FROM TABLE_B a
INNER JOIN (
SELECT
A_ID,
,MAX(RANK) AS [RANK] -- Highest Rank
FROM TABLE_B
GROUP BY A_ID
) AS b ON b.A_ID = a.A_ID AND b.RANK = a.RANK
) AS b on b.A_ID = a.id
ORDER BY a.ID ASC

Answer 5

Select A.*,D.Col_X
from
(Select C.COL_X,B.A_ID
from
(Select A_ID,MAX(rank) as MAX_rank
from TABLE_B
group by A_ID) B ----- gets the highest rank and ID of the highest rank
inner join TABLE_B c
on
concat(C.A_ID,C.RANK)= concat(B.A_ID,B.MAX_rank)) D ---- Gets the highest rank name
inner join TABLE_A A
on D.A_ID=A.ID
OUTPUT:
ID Col1 Col2 Col3 Col_X
1 c1 c2 c3 SOMEXXXX
2 c1 c2 c3 VALUE