Question

I have a table student_log, and there are multiple records for a single 'rollno'.

'sno' is the auto_increment index of the student_log table.

Suppose I want to update the value of a particular field for the last(most-recent) entry of a particular student(looked up by ‘rollno’), how do I do it? My current approach does not work. I am doing this:

update student_log set timein=current_timestamp() where rollno='ST001' and 
sno = (select sno from student_log where rollno='ST001' order by sno desc limit 1);

Using the subquery, I am trying to retrieve the sno of the most recent record where the student’s rollno matches. And I am trying to use it to match sno with the update statement, which isn’t working.

I know the syntax is correct, but I think it just means that MySQL does not allow update to use subqueries. Thanks. Ask me if I have missed out any information.

Answer 1

UPDATE student_log
SET timein=current_timestamp()
WHERE rollno='ST001'
ORDER BY sno DESC
LIMIT 1

EDIT
tested my query, and yes its posible, or what did i miss in OPs table structure

mysql> UPDATE student_log
    -> SET timein=current_timestamp()
    -> WHERE rollno='ST001'
    -> ORDER BY sno DESC
    -> LIMIT 1;
Query OK, 1 row affected, 1 warning (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT * FROM student_log;
+-----+--------+---------------------+
| sno | rollno | timein              |
+-----+--------+---------------------+
|   1 | st001  | 0000-00-00 00:00:00 |
|   2 | st002  | 0000-00-00 00:00:00 |
|   3 | st001  | 2012-07-11 12:05:23 |
+-----+--------+---------------------+
3 rows in set (0.00 sec)