How to join two Tables which have no unique ID - sql-server

Question

I am trying to Left join Table 1 to table 2
Table1 Table2
ID Data ID Data2
1 r 1 q
2 t 1 a
3 z 2 x
1 u 3 c
After i have left joined this two Tables i would like get something like this
Table1+2
ID Data Data2
1 r a
2 t x
3 z c
1 u q
and NOT
Table1+2
ID Data Data2
1 r q
2 t x
3 z c
1 u q
and my question is: is there any possibility to tell table 2 that if u have used something for table 1, dont use it and give me next value. do i have to make it im T-SQL or to and new column where i can list if this id exists then write 2 if not 1(Number data). How can i solve this problem?
Ty u in advance.

Answer 1

Since there's no uniqueness in the ID on both tables, lets add some uniqueness to it.
So it can be used to join on.
The window function ROW_NUMBER can be used for that.
An example solution that gives the expected result:
DECLARE #TestTable1 TABLE (ID INT, Data VARCHAR(1));
DECLARE #TestTable2 TABLE (ID INT, Data VARCHAR(1));
INSERT INTO #TestTable1 VALUES (1,'r'),(2,'t'),(3,'z'),(1,'u');
INSERT INTO #TestTable2 VALUES (1,'q'),(1,'a'),(2,'x'),(3,'c');
select
t1.ID, t1.Data,
t2.Data as Data2
from (
select ID, Data,
row_number() over (partition by ID order by Data) as rn
from #TestTable1
) t1
left join (
select ID, Data,
row_number() over (partition by ID order by Data) as rn
from #TestTable2
) t2 on t1.ID = t2.ID and t1.rn = t2.rn;
Note: Because of the LEFT JOIN, this does assume the amount of same ID's in table2 are equal or lower can those on table1. But you can change that to a FULL JOIN if that's not the case.
Returns :
ID Data Data2
1 r a
1 u q
2 t x
3 z c
To get the other result could have been achieved in different ways.
This is actually a more common situation.
Where one wants all from Table 1, but only get one value from Table 2 for each record of Table 1.
1) A top 1 with ties in combination with a order by rownumber()
select top 1 with ties
t1.ID, t1.Data,
t2.Data as Data2
from #TestTable1 t1
left join #TestTable2 t2 on t1.ID = t2.ID
order by row_number() over (partition by t1.ID, t1.Data order by t2.Data desc);
The top 1 with ties will only show those where the row_number() = 1
2) Using the row_number in a subquery:
select ID, Data, Data2
from (
select
t1.ID, t1.Data,
t2.Data as Data2,
row_number() over (partition by t1.ID, t1.Data order by t2.Data desc) as rn
from #TestTable1 t1
left join #TestTable2 t2 on t1.ID = t2.ID
) q
where rn = 1;
3) just a simple group by and a max :
select t1.ID, t1.Data, max(t2.Data) as Data2
from #TestTable1 t1
left join #TestTable2 t2 on t1.ID = t2.ID
group by t1.ID, t1.Data
order by 1,2;
All 3 give the same result:
ID Data Data2
1 r q
1 u q
2 t x
3 z c

Answer 2

Use ROW_NUMBER
DECLARE #Table1 TABLE (ID INT, DATA VARCHAR(10))
DECLARE #Table2 TABLE (ID INT, DATA VARCHAR(10))
INSERT INTO #Table1
VALUES
(1, 'r'),
(2, 't'),
(3, 'z'),
(1, 'u')
INSERT INTO #Table2
VALUES
(1, 'q'),
(1, 'a'),
(2, 'x'),
(3, 'c')
SELECT
A.*,
B.DATA
FROM
(SELECT *, ROW_NUMBER() OVER (PARTITION BY ID ORDER BY(SELECT NULL)) RowId FROM #Table1) A INNER JOIN
(SELECT *, ROW_NUMBER() OVER (PARTITION BY ID ORDER BY(SELECT NULL)) RowId FROM #Table2) B ON A.ID = B.ID AND
A.RowId = B.RowId