When joining tables adapt the "on" statement depending on the query results - sql-server

Question

I have 2 tables:
Table_1 with columns col_A, col_B , col_C , col_D , col_E
Table_2 with columns col_A, col_B , col_C , col_D , col_F
I would like to join them on columns col_A, col_B , col_C , col_D.
For the rows in Table_1 that do not get joined this way (as they don't have a match in Table_2), I would like to join them only on columns col_A, col_B , col_C.
If there are still rows in Table_1 that did not get joined, i would like to join them only on columns col_A, col_B.
And once that is done and there are still rows in Table_1 that did not get joined, i would like to join them only on column col_A.
I wrote the following script where i use a new table to get this result.
Is there is a more efficient way to do this? Preferably by creating a view, not a table?
create table new_table (col_A nvarchar(50) , col_B nvarchar(50) , col_C nvarchar(50)
, col_D nvarchar(50) , col_E nvarchar(50) , col_F nvarchar(50) )
go
insert into new_table
select Table_1.* , Table_2.col_F
from Table_1
inner join Table_2
on Table_1.col_A=Table_2.col_A
and Table_1.col_B=Table_2.col_B
and Table_1.col_C=Table_2.col_C
and Table_1.col_D=Table_2.col_D
go
insert into new_table
select Table_1.* , Table_2.col_F
from Table_1
inner join Table_2
on Table_1.col_A=Table_2.col_A
and Table_1.col_B=Table_2.col_B
and Table_1.col_C=Table_2.col_C
where concat (Table_1.col_A, Table_1.col_B , Table_1.col_C , Table_1.col_D , Table_1.col_E
not in (select concat (col_A, col_B , col_C , col_D , col_E) from new_table)
go
insert into new_table
select Table_1.* , Table_2.col_F
from Table_1
inner join Table_2
on Table_1.col_A=Table_2.col_A
and Table_1.col_B=Table_2.col_B
where concat (Table_1.col_A, Table_1.col_B , Table_1.col_C , Table_1.col_D , Table_1.col_E
not in (select concat (col_A, col_B , col_C , col_D , col_E) from new_table)
go
insert into new_table
select Table_1.* , Table_2.col_F
from Table_1
inner join Table_2
on Table_1.col_A=Table_2.col_A
where concat (Table_1.col_A, Table_1.col_B , Table_1.col_C , Table_1.col_D , Table_1.col_E
not in (select concat (col_A, col_B , col_C , col_D , col_E) from new_table)
go

Answer 1

You could join them on just colA and then assign some different numbers:
WITH cte AS(
SELECT
CASE WHEN t1.D = t2.D THEN 100 ELSE 0 END +
CASE WHEN t1.C = t2.C THEN 10 ELSE 0 END +
CASE WHEN t1.B = t2.B THEN 1 ELSE 0 END as whatMatched,
*
FROM
t1 JOIN t2 on t1.A = t2.A
)
Now if a row got 111 we know that all (ABCD) matched, got 0 then only A matched etc..
So we can ask for only some rows:
SELECT * FROM cte WHERE whatmatched IN (111,11,1,0)
And lastly if there were multiples (matching on just A might mean there are duplicates), we can assign a row number to them in descending order and only take the first row:
SELECT x.* FROM
(SELECT *, ROW_NUMBER() OVER(ORDER BY whatmatched DESC) rown FROM cte WHERE whatmatched IN (111,11,1,0)) x
WHERE x.rown = 1
If it suits you better to use letters
we can assess the matches, choose only A, AB, ABC, or ABCD, then pick the most specific one by looking at the LENgth of the match string:
WITH cte AS(
SELECT
'A' +
CASE WHEN t1.B = t2.B THEN 'B' ELSE '' END +
CASE WHEN t1.C = t2.C THEN 'C' ELSE '' END +
CASE WHEN t1.D = t2.D THEN 'D' ELSE '' END as whatMatched,
*
FROM
t1 JOIN t2 on t1.A = t2.A
)
SELECT x.* FROM
(SELECT *, ROW_NUMBER() OVER(ORDER BY LEN(whatmatched) DESC) rown FROM cte WHERE whatmatched IN ('A','AB','ABC','ABCD')) x
WHERE x.rown = 1
If you want ties (i.e. a row from t1 that matches two rows from t2 because their A/B/C is the same and D differs, use DENSE_RANK instead of ROW_NUMBER so they end up tied for first place