如何按顺序查找最高记录

Question

I need to join two tables using a multipart key, selecting only the records in the second table with the highest sequence numbers (equating to latest version). I'm having issues with this.

Table A Key1, Key2, Key3, data1, Data2

Table B Key1, Key2, Key3, Key4, Key5, Key_SEQ, Data1, Data2

I need to join the two tables on keya/keyb/kyc and selecting only the max sequence. the examples I have seen seem to work for single part keys, but seem unwieldly on multipart keys.

Just an update, the database is DB2i (iSeries).

Answer 1

You don't mention which SQL database you are using. The following is standard SQL and should perform well in most databases:

select *
from tablea a join
     tableb b
     on a.key1 = b.key1 and
        a.key2 = b.key2 and
        a.key3 = b.key3
where not exists (select 1
                  from tableb b2
                  where b.key1 = b2.key1 and
                        b.key2 = b2.key2 and
                        b.key3 = b2.key3 and
                        b2.key_seq > b.key_seq
                 );

Answer 2

You might be able to do it with a GROUP BY clause, but I'm not entirely sure.

SELECT * from A join B on A.key1=B.key1 AND A.key2=B.key2 AND A.key3=B.key3 AND B.Key_SEQ = MAX(B.Key_SEQ) GROUP BY B.key1, B.key2, B.key3;

Another approach is to use a left join syntax:

SELECT A.*, B.* from A join B on A.key1=B.key1 AND A.key2=B.key2 AND A.key3=B.key3 LEFT JOIN B as b2 on B.key1 = b2.key1 AND B.key2 = b2.key2 AND B.key3 = b2.key3 AND B.Key_SEQ < b2.Key_SEQ WHERE b2.Key_SEQ IS NULL;

This query basically says "Give A join B join B again, where the Second B is greater than the First B, but Second B doesn't exist." In other words, there is no B higher than the first B.