greatest-n-per-group

Table Essentially looks like: Serial-ID, ID, Date, Data, Data, Data, etc. There can be Multiple Rows for the Same ID. I'd like to create a view of this table to be used in Reports that only shows the most recent entry for each ID. It should show all of the columns. Can someone help me with the SQL select? thanks. There's about 5 different ways to do this, but here's one: SELECT * FROM yourTable AS T1 WHERE NOT EXISTS( SELECT * FROM yourTable AS T2 WHERE T2.ID = T1.ID AND T2.Date > T1.Date ) And here's another: SELECT T1.* FROM yourTable AS T1 LEFT JOIN yourTable AS T2 ON ( T2.ID = T1.ID AND T2

I'm selecting one max row per group and I'm using groupby / agg to return index values and select the rows using loc . For example, to group by "Id" and then select the row with the highest "delta" value: selected_idx = df.groupby("Id").apply(lambda df: df.delta.argmax()) selected_rows = df.loc[selected_idx, :] However, it's so slow this way. Actually, my i7/16G RAM laptop hangs when I'm using this query on 13 million rows. I have two questions for experts: How can I make this query run fast in pandas? What am I doing wrong? Why is this operation so expensive? [Update] Thank you so much for

Related to - PostgreSQL DISTINCT ON with different ORDER BY I have table purchases (product_id, purchased_at, address_id) Sample data: | id | product_id | purchased_at | address_id | | 1 | 2 | 20 Mar 2012 21:01 | 1 | | 2 | 2 | 20 Mar 2012 21:33 | 1 | | 3 | 2 | 20 Mar 2012 21:39 | 2 | | 4 | 2 | 20 Mar 2012 21:48 | 2 | The result I expect is the most recent purchased product (full row) for each address_id and that result must be sorted in descendant order by the purchased_at field: | id | product_id | purchased_at | address_id | | 4 | 2 | 20 Mar 2012 21:48 | 2 | | 2 | 2 | 20 Mar 2012 21:33 | 1 |

I have a table of sensor data. Each row has a sensor id, a timestamp, and other fields. I want to select a single row with latest timestamp for each sensor, including some of the other fields. I thought that the solution would be to group by sensor id and then order by max(timestamp) like so: SELECT sensorID,timestamp,sensorField1,sensorField2 FROM sensorTable GROUP BY sensorID ORDER BY max(timestamp); This gives me an error saying that "sensorField1 must appear in the group by clause or be used in an aggregate." What is the correct way to approach this problem? For the sake of completeness,

I have a sample table with similar structure & data as shown below: +------+---------+-------------+------------+ | S_ID | S_NAME | SUBJECT | MARK_VALUE | +------+---------+-------------+------------+ | 1 | Stud | SUB_1 | 50 | | 2 | Stud | SUB_2 | 60 | | 3 | Stud | SUB_3 | 70 | | 4 | Stud_1 | SUB_1 | 40 | | 5 | Stud_1 | SUB_2 | 50 | | 6 | Stud_2 | SUB_2 | 40 | +------+---------+-------------+------------+ Table has consolidated mark of each student in all subjects each that student has appeared. Please help me, to write a query to extract MAXIMUM mark obtained by each student ( irrespective of