Shantanu's Blog

Corporate Consultant

April 30, 2008

 

MySQL Case Study - 170

Trying to keep this simple,
I have many pickers and I am looking to find:
How many apples, how many oranges picked today and total of all fruit so far for each picker..

[picker] [apples] [oranges [date]
a 0 50 20080102
b 0 75 20080102
c 0 30 20080102
a 20 0 20080103
b 50 0 20080103
c 10 0 20080103
a 0 95 20080104
b 100 0 20080104
c 0 85 20080104

The results I am trying to get are something like what follows:
[picker] [apples] [oranges] [date] [total fruit]
a 0 95 20080104 165
b 100 0 20080104 225
c 0 85 20080104 125

I can can easily get the most current rows for each picker
select picker, apples, oranges from work where date='20080104',
I can easily get the total fruit for each picker
select picker, (sum(apples) + sum(oranges)) from work group by picker,
I just cannot seem to cram it all in one sql query.

I have looked at 'WITH ROLLUP' but it does not seem to do what I need to do.

http://forums.mysql.com/read.php?10,207297,207297#msg-207297

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MySQL Case Study - 169

Order By problem

Here is a sample set of data (ignore the dashes - - - - - - between columns):

topic_id - - - message_time

1 - - - - - - 2008-04-12 16:27:56
1 - - - - - - 2008-04-12 16:27:56
1 - - - - - - 2008-04-12 16:27:56
2 - - - - - - 2008-04-13 16:27:56
1 - - - - - - 2008-04-26 17:27:56
2 - - - - - - 2008-04-26 18:27:56
2 - - - - - - 2008-04-26 19:27:56
2 - - - - - - 2008-04-26 20:27:56

What I would like have happen is for the topics that have the most entries within the last seven days to be returned first. If there is a tie, it should go to the one with the most recent entry.

Below is the query I started with, but it doesn't seem to even be getting the first part of what I want. Any help would be greatly appreciated.

SELECT DISTINCT(topic_id) AS topic_id
FROM messages
ORDER BY COUNT(message_time) >= NOW()
LIMIT 0,25;

http://forums.mysql.com/read.php?10,207167,207167#msg-207167

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April 18, 2008

 

MySQL Case Study - 168

select rows with max value?
Given a table like this:

CREATE TABLE IF NOT EXISTS `people` (
`name` varchar(10) NOT NULL,
`age` smallint(5) unsigned NOT NULL,
PRIMARY KEY (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `people` (`name`, `age`) VALUES ('bob', 13), ('john', 25), ('mike', 25), ('steve', 8), ('sue', 13);

How would I select only the rows with the highest 'age' value?
In other words, because "john" and "mike" are the oldest, I want only their rows.

http://forums.mysql.com/read.php?10,199598,199598#msg-199598

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April 14, 2008

 

MySQL Case Study - 167

How to get the oldest N rows per user with the totol row count bigger than some threshold?

I have a log table which record user's activity.

Simple table design as follows.

CREATE TABLE log (
id int unsigned not null auto_increment,
user_id int unsigned not null,
date int unsigned not null,
...,
PRIMARY (id)
KEY (user_id, date)
) engine=innodb


I want to get the oldest N(for example count(id) - 2) rows per user whose total record count exceeds certain limit (for example 2). The N per user may vary according to the total record count per each user.

For example, the rows of table log may be as follows.
id, user_id, date,
1 1000 20071102
2 1001 20071103
3 1000 20071104
4 1000 20071101
5 1000 20071108
6 1000 20071109
7 1001 20071102
8 1001 20071109
9 1002 20071107
10 1001 20071114

I want to get the result set like following.

4 1000 20071101
1 1000 20071102
3 1000 20071104
7 1001 20071102
2 1001 20071103

normal gourp by clause will return only one row per each group.
So it is hard to do this kind of query.


select id, user_id, regdate from log as l inner join (select user_id, .. FROM log GROUP BY user_id HAVING COUNT(id) as cnt > 2) as s on (l.user_id = s.user_id) ) order by user_id, regdate limit cnt-2

This join does not achieve my result set, because limit cannot limit result set per user_id.

Is there any efficient way to achieve my purpose?

http://forums.mysql.com/read.php?24,193764,193764#msg-193764

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