How to replicate a SAS merge

Question

I have two tables, t1 and t2:

t1
  person | visit | code1 | type1
       1       1      50      50 
       1       1      50      50 
       1       2      7         


        

Answer 1

You can replicate a SAS merge by adding a row_number() to each table:

select t1.*, t2.*
from (select t1.*,
             row_number() over (partition by person, visit order by ??) as seqnum
      from t1
     ) t1 full outer join
     (select t2.*,
             row_number() over (partition by person, visit order by ??) as seqnum
      from t2
     ) t2
     on t1.person = t2.person and t1.visit = t2.visit and
        t1.seqnum = t2.seqnum;

Notes:

• The ?? means to put in the column(s) used for ordering. SAS datasets have an intrinsic order. SQL tables do not, so the ordering needs to be specified.
• You should list the columns explicitly (instead of using t1.*, t2.* in the outer query). I think SAS only includes person and visit once in the resulting dataset.

EDIT:

Note: the above produces separate values for the key columns. This is easy enough to fix:

select coalesce(t1.person, t2.person) as person,
       coalesce(t1.key, t2.key) as key,
       t1.code1, t1.type1, t2.code2, t2.type2
from (select t1.*,
             row_number() over (partition by person, visit order by ??) as seqnum
      from t1
     ) t1 full outer join
     (select t2.*,
             row_number() over (partition by person, visit order by ??) as seqnum
      from t2
     ) t2
     on t1.person = t2.person and t1.visit = t2.visit and
        t1.seqnum = t2.seqnum;

That fixes the columns issue. You can fix the copying issue by using first_value()/last_value() or by using a more complicated join condition:

select coalesce(t1.person, t2.person) as person,
       coalesce(t1.visit, t2.visit) as visit,
       t1.code1, t1.type1, t2.code2, t2.type2
from (select t1.*,
             count(*) over (partition by person, visit) as cnt,
             row_number() over (partition by person, visit order by ??) as seqnum
      from t1
     ) t1 full outer join
     (select t2.*,
             count(*) over (partition by person, visit) as cnt,
             row_number() over (partition by person, visit order by ??) as seqnum
      from t2
     ) t2
     on t1.person = t2.person and t1.visit = t2.visit and
        (t1.seqnum = t2.seqnum or
        (t1.cnt > t2.cnt and t1.seqnum > t2.seqnum and t2.seqnum = t2.cnt) or
        (t2.cnt > t1.cnt and t2.seqnum > t1.seqnum and t1.seqnum = t1.cnt)

This implements the "keep the last row" logic in a single join. Probably for performance reasons, you would want to put this into separate left joins on the original logic.