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• MySQL - Calculating fields on the fly vs storing calculated data

  - by Christian Varga

  Hi Everyone, I apologise if this has been asked before, but I can't seem to find an answer to a question that I have about calculating on the fly vs storing fields in a database. I read a few articles that suggested it was preferable to calculate when you can, but I would just like to know if that still applies to the following 2 examples. Example 1. Say you are storing data relating to a car. You store the fuel tank size in litres, and how many litres it uses per 100km. You also want to know how many KMs it can travel, which can be calculated from the tank size and economy. I see 2 ways of doing this: When a car is added or updated, calculate the amount of KMs and store this as a static field in the database. Every time a car is accessed, calculate the amount of KMs on the fly. Because the cars economy/tank size doesn't change (although it could be edited), the KMs is a pretty static value. I don't see why we would calculate it every single time the car is accessed. Wouldn't this waste cpu time as opposed to simply storing it in a separate field in the database and calculating only when a car is added or updated? My next example, which is almost an entirely different question (but on the same topic), relates to counting children. Let's say we have a app which has categories and items. We have a view where we display all the categories, and a count of all the items inside each category. Again, I'm wondering what's better. To perform a MySQL query to count all the items in each category every single time the page is accessed? Or store the count in a field in the categories table and update when an item is added / deleted? I know it is redundant to store anything that can be calculated, but I worry that calculating fields or counting records might be slow as opposed to storing the data in a field. If it's not then please let me know, I just want to learn about when to use either method. On a small scale I guess it wouldn't matter either way, but apps like Facebook, would they really count the amount of friends you have every time someone views your profile or would they just store it as a field? I'd appreciate any responses to both of these scenarios, and any resource that might explain the benefits of calculating vs storing. Thanks in advance, Christian

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• Seeking tutorial: introduction to ODBC with Delphi

  - by mawg

  I have a lot of embedded C/C++/Ada experience and an outdated smattering of Delphi plus some database stuff. Now I have to implement an app in Delphi which can manipulate MySql, Oracle, maybe MS Access. In short, I need ODBC. I need to programmatically created a database, define its structure and populate its contents, then later query its existence and programmatically search. I would prefer not to use 3rd party components, unless there is a compelling reason to do so (performance ought not to be an issue for the app, it won't have much data or be run often, at least not in v1.0) . Can anyone point me at a tutorial which can get me up to speed? Thanks

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• Hibernate/JPA DB Schema Generation Best Practices

  - by Bytecode Ninja

  I just wanted to hear the opinion of Hibernate experts about DB schema generation best practices for Hibernate/JPA based projects. Especially: What strategy to use when the project has just started? Is it recommended to let Hibernate automatically generate the schema in this phase or is it better to create the database tables manually from earliest phases of the project? Pretending that throughout the project the schema was being generated using Hibernate, is it better to disable automatic schema generation and manually create the database schema just before the system is released into production? And after the system has been released into production, what is the best practice for maintaining the entity classes and the DB schema (e.g. adding/renaming/updating columns, renaming tables, etc.)? Thanks in advance.

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• MySQL Many to Many with Unique keys and Update/Select if Exists otherwise Insert

  - by Jayrox

  In my goal to have a Many-to-Many relationship in my MySQL database I have arrived at another bridge to build. Current Tables: Users (id, name) Tags (id, name) User_Tags (user_id, tag_id) Here is the goal: I would like to have the ability to take a tag i.e: #fb and insert it into my Tags database which has a unique constraint on name and if the tag exists I would like to return it's id I would like to insert the tag.id and the current user's user.id into User_Tags. I do not have any unique constraints on the User_Tags table because I would like to be able to track the user's most commonly used tags and present them in a list ordered by frequency of use. I am currently developing this project in PHP using prepared statements. Additionally, performance is not currently an issue as my user count is still fairly small, but it is steadily growing and may be an issue in the future.

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• Python and sqlite3 - importing and exporting databases

  - by JPC

  I'm trying to write a script to import a database file. I wrote the script to export the file like so: import sqlite3 con = sqlite3.connect('../sqlite.db') with open('../dump.sql', 'w') as f: for line in con.iterdump(): f.write('%s\n' % line) Now I want to be able to import that database. I tried: import sqlite3 con = sqlite3.connect('../sqlite.db') f = open('../dump.sql','r') str = f.read() con.execute(str) but I'm not allowed to execute more than one statement. Is there a way to get it to run a .sql script directly?

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• How to compare a string column(as DateTime) in LINQ?

  - by lyon

  I have a database with a ValidDate field - it's a string(we made a mistake, it should be a datetime, but we can't modify the database now.) and now I want to compare this filed with a parameter(validDateStart) from the website: priceList = priceList.Where(p => Convert.ToDateTime(p.ValidDate) >= Convert.ToDateTime(validDateStart)); var list = initPriceList.ToList(); But I get an error: The method ToDateTime is not implemented. Can anybody give me some help? Thanks!

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• I'm looking for a constraint to prevent the insert of an empty string in MySQL

  - by Marga Keuvelaar

  Ok, in this question I learned how to prevent the insert of a NULL value. But, unfortunately, an empty string is being inserted anyway. Apart from preventing this on the PHP side, I'd like to use something like a database constraint to prevent this. Of course a check on the application side is necessary, but I'd like it to be on both sides. I am taught that whatever application is talking to your database, it should not be able to insert basically wrong data in it. So... CREATE TABLE IF NOT EXISTS tblFoo ( foo_id int(11) NOT NULL AUTO_INCREMENT, foo_test varchar(50) NOT NULL, PRIMARY KEY (foo_id) ); Would still allow me to do this insert: INSERT INTO tblFoo (foo_test) VALUES (''); Which I would like to prevent.

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• How and when to log account access login with PHP?

  - by Nazgulled

  I want to implement a basic login system in some PHP app where no cookies will be involved. I mean, the user closes the browser and the login expires, it will remain active during the browser session (or if the user explicitly logs out) otherwise. I want to log all this activity and I'm thinking that every time the user refreshes the page, opens a different link or logs out, I record that time as the last access made by that user, overwriting the previous access log. But my problem is when and how should I insert another record into the database instead of overwriting the last one? Should I just define a timeout and if the last access was made above that timeout, another log should be inserted into the database? Should the session expire too after that timeout? Or is there a better way? Ideally, I would like to log the "log out action" when the browser was closed, but I don't think there's a way to detect that is there? Suggestions?

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• Send Email Using Classic ASP with an Embeded Image

  - by Khilen

  hi i am making NewsLetter using the wysiwyg Editor.. it allows me to upload the Image Path and Image Path is stored in the Upload Directory.. Not When i retrieve that Image using it works in website.. the editor's value is stored in database example <br> hi <img src="upload/acb.gif"> <br> Hello i am sending Email and the detail of this email is received from database and this detail is sent to visitor he is gettion all text value but not able to see Image so suggest me what to do..?

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• How can I access the value of a checkbox without a postback?

  - by Starwfanatic

  I have a radgrid controlling access to an application for users. There are two columns, a 'name' column and a checkbox 'access' column representing their current access permission. the checkboxes are populated from a database. I would like the to change the checkboxes to grant or deny access to the users I specify. Since there are alot of users I would like to make multiple changes and use a submit button to write the changes to the database. I do not want to postback with each checkbox change and wait for the page to "blink". My problem is that with the checkbox postback disabled, when I click submit, the value of the checkboxes are not registering the changes I make. I need a way to access the current client-side state of the checkbox or some workaround to accomplish this. Thanks in advance!

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• in c# how would one escape a break in quotes

  - by Derek Vance

  i am trying to database some files on my computer with mysql in c#, its having an issue with VALUES ('drum'n'bass')

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• Trouble dragging and dropping gui components onto other .net forms using NHibernate

  - by IsaacB

  Hi, using VS2008, here. I have a GUI component that loads some stuff from a database mapped by nhibernate in its constructor. When I drag and drop this component onto another form from the toolbox, NHibernate complains that it can't find the config file in program files\visual studio 9\common7\ide. Why is it looking here for the cfg file? I'm actually calling database stuff through another project in the same solution, and the cfg file is located at the root of that project. copy/pasting the cfg file over to there does work, but I'm working with svn here and I don't want to have configuration files outside the repository. Something else that would help would be turning off how it tries to load the data in the form design screen. How would I do that?

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• how much time it will take to learn action script 3 in flash

  - by Mirage

  I know php mysql , javascript , jquery very well. I have never touced flash. Now have to do website in complete flash with action scripting. Keeping flash animation and scripting separate. How much time it will take for me to build e,g 1)Simple page with 1 column layout like , header , navigation bar , content. 2)The navigation items will be loaded from database using xml and flash 3)The content will also load from database depending upon navigation item clicked HOw much time forinserting flash objects and how much time for scripting. Thanks

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• Mobile App integrating with Web portal

  - by Shine

  I'm planning to develop a city info site. The web pages will display contents from a constantly updating MySQL database- most probably using a CMS. I am planning to share the same database for a Mobile app related to this. In the pages there are options for using geolocation, rate and comment for the user. I think, this mobile app should work while it's offline too. So how should i deal with data for mobile? Should i use a connection between MySQL and IndexedDB? (I'm not quite sure about this part). Appreciating your time & help.

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• From comma separated list to individual item result set. *mysql

  - by Raziel

  I'm doing some data migration from a horribly designed database to a less horribly designed database. There is a many to many relationship that has a primary key in one table that corresponds to a comma separated list in another. FK_ID | data ------------- 1,2 | foo 3 | bar 1,3,2 | blarg Is there a way to output the FK_ID field with each comma separated element as a single line in the result set? result set FK_ID | data ------------- 1 | foo 2 | foo 3 | bar 1 | blarg 2 | blarg 3 | blarg I'm thinking this would require some sort of recursive query which I don't think mysql has. Thanks in advance.

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• Syncing a table records with a Service response frequently

  - by Karthik Dheeraj

  I am requesting data from a service whose response in stored in a database.First, I have an empty table, whenever I make my very first request the records from the service comes to my database table. from now, whenever I make second request, the service will provide me some records which may be same as my first response, may be new records, may be updated records etc. my query is to how to update my table with respect to the responses coming from the service during my second request on-wards? so that Unchanged records will remain same, New records will be added, updated records will be updated.Do I need to write any stored procedure on my DB or any workaround ?what might be the scenario if I use Nomysql DB's like mongo DB ? Thanks In Advance.

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• Are there good reasons not to use an ORM?

  - by hangy

  During my apprenticeship, I have used NHibernate for some smaller projects which I mostly coded and designed on my own. Now, before starting some bigger project, the discussion arose how to design data access and whether or not to use an ORM layer. As I am still in my apprenticeship and still consider myself a beginner in enterprise programming, I did not really try to push in my opinion, which is that using an object relational mapper to the database can ease development quite a lot. The other coders in the development team are much more experienced than me, so I think I will just do what they say. :-) However, I do not completely understand two of the main reasons for not using NHibernate or a similar project: One can just build one’s own data access objects with SQL queries and copy those queries out of Microsoft SQL Server Management Studio. Debugging an ORM can be hard. So, of course I could just build my data access layer with a lot of SELECTs etc, but here I miss the advantage of automatic joins, lazy-loading proxy classes and a lower maintenance effort if a table gets a new column or a column gets renamed. (Updating numerous SELECT, INSERT and UPDATE queries vs. updating the mapping config and possibly refactoring the business classes and DTOs.) Also, using NHibernate you can run into unforeseen problems if you do not know the framework very well. That could be, for example, trusting the Table.hbm.xml where you set a string’s length to be automatically validated. However, I can also imagine similar bugs in a “simple” SqlConnection query based data access layer. Finally, are those arguments mentioned above really a good reason not to utilise an ORM for a non-trivial database based enterprise application? Are there probably other arguments they/I might have missed? (I should probably add that I think this is like the first “big” .NET/C# based application which will require teamwork. Good practices, which are seen as pretty normal on Stack Overflow, such as unit testing or continuous integration, are non-existing here up to now.)

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• InnoDB not supported by webhost. What now?

  - by Peter Perhác

  I was developing a small WAMP web application on my laptop, where I have an instance of mySQL running and I chose InnoDB for my DB engine. After several weeks' development I wanted to make it available to the public and found out the database server provided by my web host does not support InnoDB, only MyISAM. The create-and-populate script generated from the innoDB schema on my laptop, when executed against the live database, can manage to create individual TABLES but then runs into problems creating the VIEWs. Are views not supported in MyISAM? I know FOREIGN KEYs are not. That's very much why I made the choice of InnoDB... What are my chances of making my innoDB schema design work with myISAM? Is there any straightforward way of converting the whole schema from one storage engine to the other? Should I look for another web host that does provide a mysql db that supports innoDB?

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• How to Import data from Excel 2010 to SQL table

  - by user2950101

  I am using this , Insert into smst(id,mobile,day,month,year,time,model,imie1,imie2,FullMessage)select * FROM OPENROWSET('Microsoft.Ace.OLEDB.14.0','Excel 14.0;Database=L:\SMS.xlsx;HDR=YES', 'SELECT id,mobile,day,month,year,time,model,imie1,imie2,FullMessage FROM [Sheet2]') Could you please help and find the error? sql error : 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '('Microsoft.Ace.OLEDB.14.0','Excel 14.0;Database=L:\SMS.xlsx;HDR=YES', 'SELECT i' at line 1 i am using excel 2010.

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• RODBC sqlSave and column names

  - by waanders

  I've a question about using sqlSave. How maps RODBC data in the data frame to the database table columns? If I've a table with columns X and Y and a data frame with columns X and Y, RODBC puts X into X and Y into Y (I found out by trail-and-error). But can I explicitly tell R how to map data.frame columns to database table columns, like put A in X and B in Y. I'm rather new to R and think the RODBC manual is a bit cryptic. Nor can I find an example on the internet.

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• How do I set up a test duplicate of a Django and Postgresql based web application?

  - by cojadate

  Not sure if this is an excessively broad and newbie-ish question for Stack Overflow but here goes: I paid someone else to build a web application for me and now I want to tweak certain aspects of it myself. I learn best by trial and error – changing stuff and seeing what happens. Obviously that's not a great way to treat a live site, so I need to duplicate the site on some kind of test server which I can play with without fear of the consequences. Unfortunately the closest I've come to programming has been creating ActionScript-based websites. I've never touched a database. So I really don't know where to start with setting up a test server. I would really appreciate any advice about where to start. I am completely ignorant and lost here. The web application is built in python/django using a Postgresql database. I use Mac OS X 10.6 if that makes any difference.

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• Gujarati Font Rendering

  - by Vishal Khakhkhar

  I am having sqlite database containing gujarati words.. The sql query for the database is... BEGIN TRANSACTION; CREATE TABLE eng_guj (_id INTEGER PRIMARY KEY, eng_word , guj_word ); INSERT INTO eng_guj VALUES(1,'aardvark','??? ??????? ?????????? ?? ?????? ????? ??????.'); COMMIT; I want to display the text in textview.. but its not rendering properly.. meant Its displaying the word in gujarati like "???????" will be displayed as "??????". I already have used Typeface and different ttf fonts.

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• PHP/MySQL - Special characters in URLs. How to avoid?

  - by RC

  Hey everyone, My database contains information extracted from an external feed. In this raw text feed, the following text is used in place of special characters: & - &amp; ' - &39; é - &eacute; I extract some of this text to form URLs. For example, a URL that I construct from data containing these characters might look like this: http://url.com/search/?brand=Franklin&Hédgson's I use the GET variables in this URL to construct further lookups, which leads to a couple of specific problems: The é and ' characters are sent back to MySQL as they appear, and so they don't trigger any results (because the characters take the full HTML form in the database text). The & within the URL separates the variable, and the GET returns only Franklin, when it should return the whole string. Are there any straightforward ways of dealing with this? Thanks.

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• Mysqldump create empty sql file? [php & mysql on Windows]

  - by mysqllearner

  Hi all, I tried to dump a database: <?php $dbhost = "localhost"; $dbuser = "XXXX"; $dbpass = "XXXXXXXX"; $dbname = 'testdb'; $list = shell_exec ("C:\wamp\bin\mysql\mysql5.1.33\bin\mysqldump.exe $dbname --user=$dbuser--password=$dbpass > dumpfile.sql"); ?> I tried both specified full path to mysqldump.exe or just use mysqldump, it still give me a 0kb dumpfile.sql Details: Programming Language: PHP Database: MySql 5.XX OS(server): Windows Server 2003. (currently testing on Windows Vista machine) EDIT @ Jeremy Heslop: I tried: shell_exec("C:\wamp\bin\mysql\mysql5.1.33\bin\mysqldump.exe --opt -h $dbhost -u$dbuser -p$dbpass $dbname > test.sql"); safe_mode = off Still no luck man.

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• Method of documentation for SQL Stored Procedures

  - by Chapso

  I work in a location where a single person is responsible for creating and maintaining all stored procedures for SQL servers, and is the conduit between software developers and the database. There are a lot of stored procedures in place, and with a database diagram it is simple enough 90% of the time to figure out what the stored procedure needs for arguments/returns as output. For the other 10% of the time, however, it would be helpful to have a reference. Since the DBA is a busy guy (aren't we all), it would be good to have some program which documents the stored procedures to a file so that the developers can see it without being able to access the SPs themselves. The question is, does anyone know of a good program to accomplish this? Basically what we need is something that gives the name of the SP, the argument list and the output, both with datatypes and a nullable flag.

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