从两个表中选择一行的最大值

Question

Please i am trying to query data with a maximum value from two tables,

table1
|user_id | name |
| 001    | Paul |
| 002    | Sean |


table2
|id  | class | Year | user_id |
|201 |  1A   | 2010 |  001    |
|202 |  2A   | 2011 |  001    |
|203 |  1B   | 2010 |  002    |

The user_id in table2 references the user_id from table1

This is how i want my output to be

OUTPUT
 | user_id | name | class| year |
 | 001     | Paul | 2A   | 2011 |
 | 002     | Sean | 1B   | 2010 |

TRIED SO FAR

SELECT 
       a.user_id,
       a.name,
       b.class,
       max(Year) as year
FROM
       table1 a
INNER JOIN
           table2 b
    ON
           a.user_id=b.user_id

GROUP BY user_id

This query above gives me a maximum year with a different class value in a row, thus the previous class value. This is how it looks like

 | user_id | name | class| year |
 | 001     | Paul | 1A   | 2011 |
 | 002     | Sean | 1B   | 2010 |

Please where am i going wrong in my query? Any help is appreciated. Thanks

Answer 1

Maybe you could order the full result set first by year, and then group by user id:

SELECT * FROM (

SELECT 
       a.user_id,
       a.name,
       b.class,
       year
FROM
       table1 a
INNER JOIN
           table2 b
    ON
           a.user_id=b.user_id
ORDER BY year desc

) h
GROUP BY user_id

SqlFiddle demo here

Answer 2

Because you are using group by already, you can use the substring_index() / group_concat() hack:

SELECT a.user_id, a.name,
       substring_index(group_concat(b.class order by year desc), ',', 1) as maxclass
       max(Year) as year
FROM table1 a INNER JOIN
     table2 b
     ON a.user_id=b.user_id
GROUP BY a.user_id, a.name;

You can also do this without a group by , using not exists :

SELECT a.user_id, a.name, b.class, b.year
FROM table1 a INNER JOIN
     table2 b
     ON a.user_id=b.user_id
WHERE NOT EXISTS (select 1 from table2 b2 where b2.user_id = b.user_id and b2.year > b.year)

The where clause rephrases the query. It says: "Get me all rows from table2 where the same user does not have a bigger year ." This is equivalent to getting the row with the maximum year. And, this is standard SQL, which often works quite well in any database.

Answer 3

Try using a subquery

SELECT 
       a.user_id,
       a.name,
       (Select class  FROM table2 where year = max(b.year)) as class,
       max(b.Year) as year
FROM
       table1 a,table2 b
WHERE
      a.user_id=b.user_id