仅返回另一个唯一列的最高值

Question

Preferably without downloading the entire database and scanning each item, is there a MySQL query that could access this table:

__________________________________
|   ColID   |      Timestamp      |
|___________|_____________________|
|_____2_____|_2012-08-01_12:00:00_|
|_____1_____|_2012-08-01_12:01:00_|
|_____3_____|_2012-08-01_12:02:00_|
|_____3_____|_2012-08-01_12:03:00_|
|_____2_____|_2012-08-01_12:04:00_|
|_____3_____|_2012-08-01_12:05:00_|

And return only these rows:

__________________________________
|   ColID   |      Timestamp      |
|___________|_____________________|
|_____1_____|_2012-08-01_12:01:00_|
|_____2_____|_2012-08-01_12:04:00_|
|_____3_____|_2012-08-01_12:05:00_|

So as to extract only one of each ColID with the highest Timestamp

Answer 1

Use GROUP BY with MAX :

SELECT   ColID, MAX(Timestamp) AS Timestamp
FROM     tbl
GROUP BY ColID

---

Also, just a tip you may want to keep in mind for the future: if you wanted to also select other columns that might be in the same table for each maximum ColID , you cannot select it directly in the above query due to the nature of GROUP BY . You will need to wrap the query in a joined subselect joining on both the id and date columns:

SELECT b.*
FROM   tbl a
JOIN   (
       SELECT   ColID, MAX(Timestamp) AS max_timestamp
       FROM     tbl
       GROUP BY ColID
       ) b ON a.ColID = b.ColID AND a.max_timestamp = b.Timestamp

Answer 2

yes you can achive this by using GROUP BY with MAX as:

SELECT ColID, MAX(Timestamp) AS max_Timestamp      
FROM my_table
GROUP BY ColID;