I have the following tables:
tblPerson:
PersonID | Name
---------------------
1 | John Smith
2 | Jane Doe
3 | David Hoshi
tblLocation:
LocationID | Timestamp | PersonID | X | Y | Z | More Columns...
---------------------------------------------------------------
40 | Jan. 1st | 3 | 0 | 0 | 0 | More Info...
41 | Jan. 2nd | 1 | 1 | 1 | 0 | More Info...
42 | Jan. 2nd | 3 | 2 | 2 | 2 | More Info...
43 | Jan. 3rd | 3 | 4 | 4 | 4 | More Info...
44 | Jan. 5th | 2 | 0 | 0 | 0 | More Info...
I can produce an SQL query that gets the Location records for each Person like so:
SELECT LocationID, Timestamp, Name, X, Y, Z
FROM tblLocation
JOIN tblPerson
ON tblLocation.PersonID = tblPerson.PersonID;
to produce the following:
LocationID | Timestamp | Name | X | Y | Z |
--------------------------------------------------
40 | Jan. 1st | David Hoshi | 0 | 0 | 0 |
41 | Jan. 2nd | John Smith | 1 | 1 | 0 |
42 | Jan. 2nd | David Hoshi | 2 | 2 | 2 |
43 | Jan. 3rd | David Hoshi | 4 | 4 | 4 |
44 | Jan. 5th | Jane Doe | 0 | 0 | 0 |
My issue is that we're only concerned with the most recent Location record. As such, we're only really interested in the following Rows: LocationID 41, 43, and 44.
The question is: How can we query these tables to give us the most recent data on a per-person basis? What special grouping needs to happen to produce the desired result?
MySQL doesn't have ranking/analytical/windowing functionality.
SELECT tl.locationid, tl.timestamp, tp.name, X, Y, Z
FROM tblPerson tp
JOIN tblLocation tl ON tl.personid = tp.personid
JOIN (SELECT t.personid,
MAX(t.timestamp) AS max_date
FROM tblLocation t
GROUP BY t.personid) x ON x.personid = tl.personid
AND x.max_date = tl.timestamp
SQL Server 2005+ and Oracle 9i+ support analytics, so you could use:
SELECT x.locationid, x.timestamp, x.name, x.X, x.Y, x.Z
FROM (SELECT tl.locationid, tl.timestamp, tp.name, X, Y, Z,
ROW_NUMBER() OVER (PARTITION BY tp.name ORDER BY tl.timestamp DESC) AS rank
FROM tblPerson tp
JOIN tblLocation tl ON tl.personid = tp.personid) x
WHERE x.rank = 1
Using a variable to get same as ROW_NUMBER functionality on MySQL:
SELECT x.locationid, x.timestamp, x.name, x.X, x.Y, x.Z
FROM (SELECT tl.locationid, tl.timestamp, tp.name, X, Y, Z,
CASE
WHEN @name != t.name THEN
@rownum := 1
ELSE @rownum := @rownum + 1
END AS rank,
@name := tp.name
FROM tblLocation tl
JOIN tblPerson tp ON tp.personid = tl.personid
JOIN (SELECT @rownum := NULL, @name := '') r
ORDER BY tp.name, tl.timestamp DESC) x
WHERE x.rank = 1