Question

I’ve got four tables. The structure of these tables is shown below (I am only showing the relevant column names).

User (user_id)
User_RecordType (user_id, recordType_id)
RecordType (recordType_id)
Record (recordType_id, record_timestamp, record_value)

I need to find the most recent record_value for each RecordType that a given user has access to. Timestamps are stored as seconds since the epoch.

I can get the RecordTypes that the user has access to with the query:

SELECT recordType_id 
FROM User, User_RecordType, RecordType 
WHERE User.user_id=User_RecordType.user_id 
AND User_RecordType.recordType_id=RecordType.recordType_id;

What this query doesn’t do is also fetch the most recent Record for each RecordType that the user has access to. Ideally, I’d like to do this all in a single query and without using any stored procedures.

So, can somebody please lend me some of their SQL-fu? Thanks!

Answer 1

SELECT
  Record.recordType_id,
  Record.record_value
FROM
  Record
INNER JOIN
  (
  SELECT
    recordType_id,
    MAX(record_timestamp) AS `record_timestamp`
  FROM
    Record
  GROUP BY
    recordType_id
  ) max_values
ON
  max_values.recordType_id = Record.recordType_id 
AND 
  max_values.record_timestamp = Record.record_timestamp
INNER JOIN
  User_RecordType
ON
  UserRecordType.recordType_id = RecordType.recordType_id
WHERE
  User_RecordType.user_id = ?