MySQL-选择-最大-分组-效率

Question

I have a question regarding performance of the MySQL DBMS. Perhaps a trivial matter. There are two tables and I need to get result as below :

     PLAYERS                           VISITS
     ID | PLAYER_NAME                  ID | PLAYER_ID | SEEN
     ----------------                  ---------------------------
     1  | user 1                        1 | 2         | 2012-12-12
     2  | user 2                        2 | 2         | 2012-12-13
     3  | user 3                        3 | 3         | 2012-12-13
     4  | user 4                        4 | 3         | 2012-12-14
                                        5 | 3         | 2012-12-14
                                        6 | 2         | 2012-12-15
RESULT:
ID | PLAYER_NAME | LAST_SEEN  
----------------------------
 1 | user 1      | NULL / 'NEVER'
 2 | user 2      | 2012-12-15
 3 | user 3      | 2012-12-14
 4 | user 4      | NULL / 'NEVER'

My current query is :

   SELECT 
   players.id, 
   players.player_name, 
   MAX(visits.seen) AS last_seen
   FROM players
   LEFT JOIN visits ON players.id = visits.player_id
   GROUP BY players.id,players.player_name 

Works for me but it seems to me that it should be a more efficient method.

It's just key part of a larger query.

Thomas

Answer 1

To make this join efficient there has to be an index on player_id in VISITS . Look into

`CREATE INDEX` 

here.

To check the query efficiency you can always use:

EXPLAIN SELECT /* your select here */

Also if PLAYERS.ID is unique and primary key it's perfectly fine to group only by this id.

SELECT 
   players.id, 
   players.player_name, 
   MAX(visits.seen) AS last_seen
FROM players
   LEFT JOIN visits ON players.id = visits.player_id
GROUP BY players.id

but your original query is perfectly fine. Make sure you fully understand GROUP BY and the consequences for columns not included in GROUP BY if you omit columns. This can have unintended consequences in other queries (where the same id doesn't mean the same name ie)