右连接无法正常工作

Question

I have two tables as follow:

when I use the following command I get the following result:

SELECT A.enid AS enid, sum(A.comment) AS Comments, B.enname
FROM ee.entity_epoch A
right JOIN ee.entity B
ON A.id = B.enid group by A.enid

Result:

But according to the following link:

joins since I use right join I expect to get the records for saman and reza with column of comment as null. I am so confused can anyone says how can I get records with saman and reza and null for comment column plus the result shown above?

Answer 1

reza and saman both have matches in A, so no "Comments is NULL" records from A are generated for them. Are you sure you didn't want

SELECT A.enid AS enid, sum(A.comment) AS Comments, B.enname FROM ee.entity_epoch A right JOIN ee.entity B ON A.enid = B.enid group by A.enid

?

Answer 2

I think you want a subquery here before the join.

SELECT *
  FROM (SELECT enid
             , SUM(COMMENT) AS Comments
          FROM entity_epoch
          GROUP BY enid) a
        RIGHT JOIN
        entity B ON A.enid = B.enid

sqlfiddle

Personally I would reorder and make it a left join for readability, but it doesn't make any functional difference.

This can also be done as:

SELECT A.enid AS enid
      ,SUM(A.Comment) Comments
      , B.enname
  FROM entity_epoch A
       RIGHT JOIN 
       entity B
       ON A.enid = B.enid 
GROUP BY b.enid

sqlfiddlee

I'd be curiosu to see the different in exectuon plan, but don't have MySQL available.