SAS Enterprise: left join and right join difference? - sql

Question

I joined a new company that uses SAS Enterprise Guide.
I have 2 tables, table A has 100 row, and table B has over 30M rows (50-60 columns).
I tried to do a right join from A (100) to B (30M), it took over 2 hours and no result come back. I want to ask, will it help if I do a left join? I used the GUI and created the following query.
30M Record <- 100 Record ?
or
100 Record -> 30M Record ?
PROC SQL;
CREATE TABLE WORK.QUERY_FOR_CASE_NUMBER AS
SELECT t2.EMPGRPCOM,
t2.SEQINVNUM,
t2.SBSID,
t2.SBSLASTNAME,
t2.SBSFIRSTNAME,
t2.PMTDUEDATE,
t2.PREMAMT,
t2.ITEMDESC,
t2.EFFDATE,
t2.PAYAMT,
t2.MCAIDRATECD,
t2.REBILLIND,
t2.BILLTYPE
FROM WORK.'CASE NUMBER'n t1
LEFT JOIN DW.BILLING t2 ON (t1.CaseNumber = t2.SBSID)
WHERE t2.LOB = 'MD' AND t2.PMTDUEDATE BETWEEN '1Jan2015:0:0:0'dt AND '31Dec2017:0:0:0'dt AND t2.SITEID = '0001';
QUIT;

Answer 1

Left join and Right join, all other things aside, are equivalent - if you implement them the same way, anyway. I.E.,
select a.*
from a
left join
b
on a.id=b.id
;
vs
select a.*
from b
right join
a
on b.id=a.id
;
Same exact query, no difference, same time used. SQL is an interpreted language, meaning the SQL interpreter looks at what you send it and figures out what the best way to do it is - so it sees both queries and knows in both cases to do the same thing.
You can read about this in all sorts of articles, this one is a good starting point, or if that link ages just search for "right join vs left join".
Now, what you might want to consider is writing this in a different way, namely not using SQL; this kind of query SQL should be good at but sometimes isn't for some reason. I would write it as a hash table search, where the smaller case_number dataset is loaded to memory, then data step iterate over the larger table and check if it's found in the smaller dataset - if so, then great, return it.
I'd also think about whether left/right join is what you want, vs. inner join. Seems to me that if you're returning solely t2 values, right/left join isn't correct (when t1 is the "primary"): you'll just get empty rows for the non-matches. Either return a t1 variable, or use inner join.