Question

I am using mysql and I have a table with columns: postid, userid, datestamp

I want to create a new column called ‘entry’ that numbers the entries chronologically according to datestamp

So, for the rows:

3705    1   2003-12-08 13:42:13
3711    15  2003-11-12 15:22:01
3701    2   2004-01-11 01:22:12

I want to end up with:

3705    1   2003-12-08 13:42:13 2
3711    15  2003-11-12 15:22:01 1
3701    2   2004-01-11 01:22:12 3

How can I perform this task?

Answer 1

ALTER TABLE tbl ADD COLUMN nr integer;

SET @rn := 0;

UPDATE tbl
SET    rn = (@rn := @rn + 1)
ORDER  BY datestamp, postid, userid;

Here is a working demo.

I took the substitute for the missing window function row_number() in MySQL from @OMG Ponies’ Posting.
Sort by postid and userid in addition to datestamp as the timestamp column is not guaranteed to be unique.
Read about setting variables in the manual.