MYSQL Query locking up server - php

Question

When trying to execute this query my mysql server cpu usage goes to 100% and the page just stalls. I setup an index on (Client_Code, Date_Time, Time_Stamp, Activity_Code, Employee_Name, ID_Transaction) it doesn't seem to help. What steps can I go about next to fix this issue? Also there is already one index on the database if that matters any. Thanks
Here is what this query does
Database info
ID_Transaction | Client_Code | Employee_Name | Date_Time |Time_Stamp| Activity_Code
1 | 00001 | Eric | 11/15/10| 7:30AM | 00023
2 | 00001 | Jerry | 11/15/10| 8:30AM | 00033
3 | 00002 | Amy | 11/15/10| 9:45AM | 00034
4 | 00003 | Jim | 11/15/10| 10:30AM | 00063
5 | 00003 | Ryan | 11/15/10 | 12:00PM | 00063
6 | 00003 | bill | 11/14/10 | 1:00pm | 00054
7 | 00004 | Jim | 11/15/10 | 1:00pm | 00045
8 | 00005 | Jim | 11/15/10| 10:00 AM| 00045
The query takes the info above and counts it like so. By the most recent entry for each client_code. In this case the query would look like this. After php.
Jerry = 1
2 | 00001 | Jerry | 11/15/10| 8:30AM | 00033
Amy = 1
3 | 00002 | Amy | 11/15/10| 9:45AM | 00034
Ryan = 1
5 | 00003 | Ryan | 11/15/10 | 12:00PM | 00063
Jim = 2
7 | 00004 | Jim | 11/15/10 | 1:00pm | 00045
8 | 00005 | Jim | 11/15/10| 10:00 AM| 00045
$sql = "SELECT m.Employee_Name, count(m.ID_Transaction)
FROM ( SELECT DISTINCT Client_Code FROM Transaction)
md JOIN Transaction m ON
m.ID_Transaction = ( SELECT
ID_Transaction FROM Transaction mi
WHERE mi.Client_Code = md.Client_Code AND Date_Time=CURdate() AND Time_Stamp!='' AND
Activity_Code!='000001'
ORDER BY m.Employee_Name DESC, mi.Client_Code DESC, mi.Date_Time DESC,
mi.ID_Transaction DESC LIMIT 1 )
group by m.Employee_Name";
Is there a better way to write this query so it doesnt bog down my system? The query works fine with 10 database entries but it locks my server up when the database has 300,000 entries.
Thanks
Eric
+----+--------------------+-------------+--------+------------------------+--------------+---------+----------------+------+----------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------------+--------+------------------------+--------------+---------+----------------+------+----------+----------------------------------------------+
| 1 | PRIMARY | <derived2> | ALL | [NULL] | [NULL] | [NULL] | [NULL] | 8 | 100.00 | Using temporary; Using filesort |
| 1 | PRIMARY | m | index | [NULL] | search index | 924 | [NULL] | 21 | 100.00 | Using where; Using index; Using join buffer |
| 3 | DEPENDENT SUBQUERY | mi | ref | search index,secondary | search index | 18 | md.Client_Code | 3 | 100.00 | Using where; Using temporary; Using filesort |
| 2 | DERIVED | Transaction | index | [NULL] | secondary | 918 | [NULL] | 21 | 38.10 | Using index |
+----+--------------------+-------------+--------+------------------------+--------------+---------+----------------+------+----------+----------------------------------------------+

Answer 1

What about going with multiple GROUP BY's instead of the all the sub queries to simplify things.... something like:
SELECT * FROM Transaction WHERE Date_Time=CURdate() AND Time_Stamp!='' AND Activity_Code != '000001' GROUP BY Client_Code, Employee_Name
If I'm understanding your query correctly then something like this would solve the issues and prevent the need for sub queries.

Answer 2

You'll definitely want to do a join instead of a sub select.
Also, how many records are you viewing? Is pagination and using limit out of the question?
If you set up your initial query modified with inner/outer joins as a view and it doesn't crash, you'll be one step closer. Once the view is set up, you'll be able to use a much less complicated select statement - potentially paginated.