从联接表中获取最大日期

Question

My first table is a members table:

member_id,  name,   address,    email
1           jon     122 any     jon@test.ca
2           amy     111 gee     amy@test.ca

My second table is test levels accomplished:

MemberHistoryTable
member_id,  level,  date
1           a       2007
1           b       2008
1           c       2009
1           d       2010
2           a       2007
2           b       2008
2           c       2009
2           d       2010
2           d       2011

I would like to retrive the member information and the most recent date in the history table:

jon, 122 any, jon@test.ca, 2010
amy, 111 gee, amy@test.ca, 2011

I have tried many different ways of joining the two tables, such as:

SELECT
  memberTable.name,
  memberTable.address,
  memberTable.email,
  memberHistoryTable.date
FROM memberTable

LEFT JOIN memberHistoryTable
  ON memberHistoryTable.member_id=memberTable.member_id

But this reproduces multiple rows of the same information, one row for each date...

jon, 122 any, jon@test.ca, 2007
jon, 122 any, jon@test.ca, 2008
jon, 122 any, jon@test.ca, 2009
jon, 122 any, jon@test.ca, 2010
amy, 111 gee, amy@test.ca, 2007  
amy, 111 gee, amy@test.ca, 2008
amy, 111 gee, amy@test.ca, 2009
amy, 111 gee, amy@test.ca, 2010
amy, 111 gee, amy@test.ca, 2011

If I change the join to:

LEFT JOIN (SELECT MAX(memberHistoryTable.date) FROM memberHistoryTable) X
  ON x.member_id = memberTable.member_id

I get the rows I want, but the date is NULL...

    jon, 122 any, jon@test.ca, NULL
    amy, 111 gee, amy@test.ca, NULL

I have tried many variations of joins and none seem to work. Either a null value or the system just hangs.

Many hours have been spent searching and I have found many solutions that do not seem to work for me. Strangely, I need to pull the date from another table for this query, but that one seems to work.

Answer 1

You could use MAX() and a GROUP BY to get your desired result:

SELECT m.name, m.address, m.email, MAX(mh.date) `date` FROM memberTable m
INNER JOIN memberHistoryTable mh ON mh.member_id = m.member_id 
GROUP BY m.name;

I'm assuming the member_id for amy in your members table should be 2 instead of 1 , since it seems like member_id is an unique value identifying each member in your table.

The above query would yield something like:

+------+---------+-------------+--------------+
| name | address | email       | date         |
+------+---------+-------------+--------------+
| amy  | 111 gee | amy@test.ca | 2011         |
| jon  | 122 any | jon@test.ca | 2010         |
+------+---------+-------------+--------------+

I also like to point out that date is a keyword in mysql and even if it may be used as an unquoted identifier I would consider naming my column to something else.

I would also suggest that you group by member_id instead of name , since name isn't guaranteed to be unique as I understand it:

SELECT m.member_id, m.name, m.address, m.email, MAX(mh.date) `date` FROM memberTable m
INNER JOIN memberHistoryTable mh ON mh.member_id = m.member_id 
GROUP BY m.member_id;