right join versus left join

Question

In this case, a left join is the same as a right join?

mysql> 
mysql> 
mysql> use usenet;show tables;describe ARTICLE;describe NEWSGROUP;
Database chang         


        

Answer 1

Not really, because RIGHT JOIN and LEFT JOIN are symmetric. That is:

A LEFT JOIN B = B RIGHT JOIN A

RIGHT JOIN is merely syntactic sugar. The optimizer can rewrite a right join to a left join:

mysql> explain extended select * from t right join t t2 using (c1)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t2
         type: index
possible_keys: NULL
          key: c2
      key_len: 5
          ref: NULL
         rows: 4201
     filtered: 100.00
        Extra: Using index
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
         type: eq_ref
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 4
          ref: test.t2.c1
         rows: 1
     filtered: 100.00
        Extra: 
2 rows in set, 1 warning (0.00 sec)

Note the LEFT JOIN in the optimizer re-write (the tables are swapped):

mysql> show warnings\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: select `test`.`t2`.`c1` AS `c1`,`test`.`t2`.`c2` AS `c2`,`test`.`t`.`c2` AS
`c2` from `test`.`t` `t2` left join `test`.`t` on((`test`.`t`.`c1` = `test`.`t2`.`c1`))  where 1
1 row in set (0.00 sec) 

Notice that (A RIGHT JOIN B != A LEFT JOIN B) unless (A INNER JOIN B = A LEFT JOIN B). This is because A RIGHT JOIN B is not symmetrical with A LEFT JOIN B (it is symmetrical with B LEFT JOIN A).

In your case, A RIGHT JOIN B will be the same as A LEFT JOIN B unless there are NULL values in the columns you are joining. If there are NULL values, then A LEFT JOIN B will NOT be the same as A RIGHT JOIN B. If you add new articles without adding the associated newsgroup (or vice-versa) then the results would change too.

Answer 2

Codeproject has this image which explains the simple basics of SQL joins, taken from: http://www.codeproject.com/KB/database/Visual_SQL_Joins.aspx SQL joins explained

Answer 3

With your current data, yes they are the same. However as NEWSGROUP_ID is nullable then they could be different as the data changes.

Personally I always use LEFT JOINs if possible (from primary table to child table), in fact I have only needed to use a RIGHT JOIN a couple of times in over 6 years of SQL development!