Shantanu's Blog

Corporate Consultant

July 25, 2005


MySQL Case Study - 43

How to speed up the query where OR is being used?

select * from table where (
( field1 = 'VALUE1' and field2 like 'VALUE2%' )
( field3 = 'VALUE1' and field2 like 'VALUE2%' )

I have created two composite indexes - one on field1 field2 and one on field3 field2. The query takes an age to run and looking at my log indicates a full table scan.

I have also tried indexing just field1 and field3 separately but this
doesn't help.

July 19, 2005


MySQL Case Study - 42

I have a table:
create table example(time datetime, username varchar(255));

time username
2005-06-21 15:58:02 user1
2005-06-21 14:58:02 user1
2005-06-21 11:57:51 user2
2005-06-21 10:57:51 user1
2005-06-21 09:57:51 user1

The query:
select COUNT(*), username, MAX(time) as maxtime from example group by
username order by maxtime desc;

groups by username, and returns:

COUNT(*) username maxtime
4 user1 2005-06-21 15:58:02
1 user2 2005-06-21 11:57:51

I want it, however, to return:

COUNT(*) username maxtime
2 user1 2005-06-21 15:58:02
1 user2 2005-06-21 11:57:51
2 user1 2005-06-21 10:57:51

That is, do not group entries by the same user together if another
user visited in between. Is this possible?

July 14, 2005


Calling an end to poverty |

A recent study by London Business School found that, in a typical developing country, a rise of ten mobile phones per 100 people boosts GDP growth by 0.6 percentage points. Mobile phones are, in short, a classic example of technology that helps people help themselves.

July 13, 2005


MySQL Case Study - 41

I have a table that has 3 fields: id (auto-increment, primary key), parentId (integer, default 0) and sum (double)

"Normal" rows just have and id and a sum, but then there can also be childs to those rows, and child rows have the parent row's id set in the parentId-field


id | parenId | sum
1 0 120.00
2 0 23.00
3 1 12.00
4 1 20.00
5 4 10.00

So in this example rows 1 and 2 are top-level rows. Rows 3,4 are childs to row 1 and row 5 is child to row 4. The child rows' sums are always <= compared with the parent row.

I'm trying to build a select query that selects all the info in the table. My problem is that if a row has childs, I want the sum of child row's sum to be reduced from the parent row's sum. eg when returning row 1, it's sum should be 88.00 (120.00-12.00-20.00). Row 4's sum should be 10.00 (20-10).

Can I do the subtraction using a subquery?


MySQL Case Study - 40

review_id relates the vote to a specific review.

table 1 (reviews)
id, title, message
12 | John's Review of Something | The review goes here

table 2 (votes)
id, review_id, vote

3 | 12 | 6
4 | 12 | 10
8 | 12 | 8

the vote column is between 1 and 10 inclusive. In this case the total vote average for review_id 12 is (6+10+8)/3=8 and there might be another whose average is say 8.5. I want to be able to figure out which has the highest.

What I want is to be able to retrieve the id and title etc of the item in reviews with the highest avg votes.

July 07, 2005


View Limitations

No subqueries in the from clause

MySQL have recognised that this particular problem is a design flaw. The select statement (while creating a view) cannot contain a subquery in the from clause, it is possible however to use subqueries in other parts of the view definition.

create view v_emp as select * from (select emp_id from emps) as emps;
ERROR 1349 (HY000): View’s SELECT contains a subquery in the FROM clause

create view v_emps as select * from emps where dept_id in
(select dept_id from dept);
Query OK, 0 rows affected (0.03 sec)"

July 05, 2005


Free Mysql 5.0
* You will need to register (free) before you can start using.
* Can be used for critical business applications.

Frequently Asked Questions...

I want to access my database account from a script which is hosted at my Internet Service Provider. Which hostname must I use?

Ans: The hostname is (Attention: NOT!)
And your username, password and database name.

What can I enter in the field 'IP address'?

Ans: You can restrict access to your database account to a specific IP address or host to increase the security of your account.
For example, if you only want to access your database account via db4free's phpMyAdmin, you can enter 'localhost'. Or if you want to limit access to your PHP files that are hosted at your ISP, you can enter your ISP's hostname or IP address. Entering a more specific value than the percent sign (which gives you access from any host), makes it impossible to access your account from anywhere else (including hackers).

How many database accounts can I have?
Ans: You can create 2 database accounts per email address.


MySQL Case Study - 39

I'm looking to list the number of Users who have an entry matching 'Page View' in the table UserActions. The query I'm using is:

SELECT COUNT(*) AS Download_Ruequests
FROM UserActions,
FROM UserActions
WHERE UserActions.ACTION = "Request Download"
GROUP BY UserActions.UserID) AS Downloads

However, it involves 2 subqueries and I can't help but feel there must be a simpler way...
I want to return users who have viewed a page AND requested a download, but each of these actions is in a different row in the table. So I'm effectively saying, show me the User IDs which have this action and another action in a different row.
Any pointers greatly appreciated.


FROM UserActions ua
FROM UserActions
WHERE ACTION = 'Request Download') AS uad
ON ua.UserID = aud.UserID
FROM UserActions
WHERE ACTION = 'Page View') AS uap
ON ua.UserID = aup.UserID


MySQL Case Study - 38

First of all this is what i have so far

SELECT s_c.s_id, s_c.course_id, s_c.start_date, s_c.complete_date, s_c.due_date, count(s_c_u.unit_id) AS total_units, count(s_c_u.complete_date) AS complete_units
FROM s_c
ON s_c.course_id = s_c_u.course_id
WHERE s_c.s_id = xxxx
GROUP BY s_c.course_id

This selects the desired fields from table s_c (a table that contains which student does which course) then computes the total units (from the s_c_u table) that the student has in the corrisponding course and how many they have completed. But what i'd also like to do is figure out how many units are overdue based on a date that i feed in. There is a field due_date in the s_c_u table. So i need something like

[Pseudo code]
count(s_c_u.due_date) AS overdue_units WHERE due_date < date

but im not sure how to write such a query that integrates into the query above. I hope it's possible, otherwise i'll have to do it with php and it wont be very efficient.

July 02, 2005


MySQL Case Study - 37

I have a db with this table

CREATE TABLE `customers` (
`idcustomer` int(10) unsigned NOT NULL auto_increment,
`Customer Name` varchar(45) NOT NULL default '',
PRIMARY KEY (`idcustomer`)

INSERT INTO `customers` (`idcustomer`, `Customer Name`) VALUES (NULL, 'OLDCustomer');

If I execute this operation

lock table customers WRITE;
UPDATE `customers` SET `Customer Name`='MODIFYcustomer' WHERE `idcustomer`=1;
unlock table;

SELECT * FROM `customers`;
'idcustomer','Customer Name'

The record changes. Is it correct? Or is it a bug?

If I execute the same operation with a different order the record doesen't change

lock table customers WRITE;
UPDATE `customers` SET `Customer Name`='AAAAAcustomer' WHERE `idcustomer`=1;
unlock table;

SELECT * FROM `customers`;
'idcustomer','Customer Name'


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