Question

I’ve gotta question for you, I’m getting hard times trying to combine two tables, I can’t manage to find the correct query.

I have two tables:
T1: 1column, Has X records
T2: 1column, Has Y records
Note: Y could never be greater than X but it often lesser than this one

I want to join those tables in order to have a table with two columns
t3: ColumnFromT1, columnFromT2.

When Y is lesser than X, the T2 field values gets repeated and are spread over all my other values, but I want to get NULL when ALL the columns from T2 are used.

How could I achieve that?
Thanks

Answer 1

You could give each table a row number in a subquery. Then you can left join on that row number. To recycle rows from the second table, take the modulus % of the first table’s row number.

Example:

select  Sub1.col1
,       Sub2.col1
from    (
        select  row_number() over (order by col1) as rn
        ,       *
        from    @T1
        ) Sub1
left join
        (
        select  row_number() over (order by col1) as rn
        ,       *
        from    @T2
        ) Sub2
on      (Sub1.rn - 1) % (select count(*) from @T2) + 1 = Sub2.rn

Test data:

declare @t1 table (col1 int)
declare @t2 table (col1 datetime)

insert @t1 values (1), (2), (3), (4), (5)
insert @t2 values ('2010-01-01'), ('2012-02-02')

This prints:

1        2010-01-01
2        2012-02-02
3        2010-01-01
4        2012-02-02
5        2010-01-01