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  • Java: Making concurrent MySQL queries from multiple clients synchronised

    - by Misha Gale
    I work at a gaming cybercafe, and we've got a system here (smartlaunch) which keeps track of game licenses. I've written a program which interfaces with this system (actually, with it's backend MySQL database). The program is meant to be run on a client PC and (1) query the database to select an unused license from the pool available, then (2) mark this license as in use by the client PC. The problem is, I've got a concurrency bug. The program is meant to be launched simultaneously on multiple machines, and when this happens, some machines often try and acquire the same license. I think that this is because steps (1) and (2) are not synchronised, i.e. one program determines that license #5 is available and selects it, but before it can mark #5 as in use another copy of the program on another PC tries to grab that same license. I've tried to solve this problem by using transactions and table locking, but it doesn't seem to make any difference - Am I doing this right? Here follows the code in question: public LicenseKey Acquire() throws SmartLaunchException, SQLException { Connection conn = SmartLaunchDB.getConnection(); int PCID = SmartLaunchDB.getCurrentPCID(); conn.createStatement().execute("LOCK TABLE `licensekeys` WRITE"); String sql = "SELECT * FROM `licensekeys` WHERE `InUseByPC` = 0 AND LicenseSetupID = ? ORDER BY `ID` DESC LIMIT 1"; PreparedStatement statement = conn.prepareStatement(sql); statement.setInt(1, this.id); ResultSet results = statement.executeQuery(); if (results.next()) { int licenseID = results.getInt("ID"); sql = "UPDATE `licensekeys` SET `InUseByPC` = ? WHERE `ID` = ?"; statement = conn.prepareStatement(sql); statement.setInt(1, PCID); statement.setInt(2, licenseID); statement.executeUpdate(); statement.close(); conn.commit(); conn.createStatement().execute("UNLOCK TABLES"); return new LicenseKey(results.getInt("ID"), this, results.getString("LicenseKey"), results.getInt("LicenseKeyType")); } else { throw new SmartLaunchException("All licenses of type " + this.name + "are in use"); } }

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  • Linq to SQL: how get row security between write access??

    - by Francisco
    I would like to allow two threads to write in a table at the same time (I know the problem of updating the same row, but this would be a story apart). I need that in behalf of speed up the operations in my aplication (one thread could write in row X while another could do the same in row X+n instead of waiting the first to finalize). So, can I block rows instead of tables with Linq to SQL? Thanks.

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  • Help with MySQL date operation

    - by jartaud
    Hello i have two columns: duration (int) and dateCol (date) in a table. How should i proceed if i want to know when CURDATE() - dateCol will iqual to duration. ie: CURDATE() - dateCol (2010-05-08) = duration (14 days), and then send an e-amil?

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  • question about group by

    - by chun
    Hi in mysql how to write a sql like this, to get the amount of X 20 and <20 select date, numberOfXMoreThan20,numberOfXLessThan20, otherValues from table group by (date, X20 and X<20) thanks

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  • Advantage Database Server: in-memory queries.

    - by ie
    As far as I know, ADS v.10 tries to keep result of query in memory until it is a quite huge. The same should be true for the __output table and for temporary tables. When the result becoming large, swapping stated. The question is what memory limit is set for a query, a worker, whatever? Could this limit be configured? Thanks.

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  • .net databound lists

    - by d daly
    Hi, Is there any way around this? I have a few dropdown lists bound to lookup tables in sql server. some old records imported from a previos version of the system wont open due to data in these fields not matching the current dropdown data. Other than adding the old data to the lookup table (which I dont want to do) is there a way around this? Thanks DD

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  • hibernate bulkupdate: update query syntax

    - by QuanNH
    when i update a table using hibernate getHibernateTemplate().bulkUpdate("UPDATE Channel SET number = 40 AND active = 0"); i get error: ERROR [PARSER]: unexpected token: AND evething run well if i remove AND active = 0 i dont know how to correct this query. help me pls, thanks :)

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  • WordPress Business Directory - best approach

    - by NTulip
    I want to implement a business directory in WordPress and I am looking for feedback on the best approach: I have a categories and a businesses table Do I create a page for every business together with it's category relationship Do I create a page and assign it a template? What are the ups and downs with each approach? Looking for some answers from people that might of already done this and can speak from experience

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  • How do I update a cumulative field in a Rails database (using ActiveRecord or Mongoid)?

    - by picardo
    I want to update a field in a database table that has to have a cumulative value. So basically I need to find the current value of the field and update it using a new number. My first inefficient try at this (in Mongoid) is: v = Landlord.where(:name=>"Lorem") v.update_attributes(:violations=>v.violations + 10) Is there a simple method than making one query to read, then sum up, and another query to write?

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  • SQL Server Compact Edition - Create Database Tables

    - by Lennie De Villiers
    Hi, I got it right to create a SQL Server CE database table using the code below: string connectionString = "DataSource=\"test.sdf\"; Password=\"mypassword\""; SqlCeEngine en = new SqlCeEngine(connectionString); en.CreateDatabase(); But how do I create my database tables and insert data? I've got the SQL created statements stored in the application Resource location.

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  • Rearranged columns in C#

    - by chown
    How can I get the rearranged columns out of a datagridview in C#? I need to get all the columns currently being displayed in order. I can get the default columns order by setting datagridview.AutoGenerateColumns=false but I get the same old order even after I've rearranged the columns in my table. Thanks

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  • Core Data Change property value when another property changes

    - by user320587
    Hi, I have a Core Data Entity which has three properties startDate, endDate and duration. All three properties are persistent properties. I would like to know how I can calculate and update the duration property whenever the value for startDate and endDate changes? BTW, I won't be able to make the duration as transient property since I have to use the property for sorting in my table view? Any help is greatly appreciated Thanks, Javid

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  • How to check user id already exists

    - by Sheery
    Hi Friends, I am a beginner coder, i am building a project using C# Asp.Net in which i am registering users with a user id, now my question is that how to check that the user id is already exists in the user table or not when user trying to register, i am using sql server 2000?

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  • CakePHP - Paginate / Query: find lowest of multiple fields & order by it

    - by Atea
    I am using CakePHP for a price-comparison website. I have a table products with fields: id, pride_regular, price_action. I would like to combine the fields price_regular and price_action into a dynamic field: price. The lowest value of these two fields should be the value of the new price field. Also I want to order on it Ascending. Should I use a custom MySQL-query?

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  • LINQ 2 SQL:Left outer join for my SQL statement

    - by Shyju
    Can any one tell me the LINQ 2 SQL version (in vb.net) of the below Left outer join query.I am trying to get all Employees with name "Shyju" and their address line 1 if it exist in the address table SELECT E.EMPLOYEE_NAME, E.AGE,A.ADRESS_LINE1 FROM EMPLOYEE_MASTER E LEFT OUTER JOIN ADDRESS_MASTER A ON E.ID=A.EMPLOYEE_ID WHERE E.EMPLOYEE_NAME='Shyju' Thanks in advance

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  • Return dataset in dataflow

    - by praveen
    Hi All, Could I get ideas on retrieving the dataset using lookup method. Basically, my scenario as I have source data needs to lookup for other source table and on matching column from source I need to get all the records from other source data. its a one to many relations. I tried Lookup but gives only one record on matching condition, OLE DB command don't retrieve any data as it will do only Insert/Update operations. Thanks prav

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  • How to solve - Illegal mix of collations in mysql?

    - by rocksolid
    Am getting the below error when trying to do a select through a Stored procedure in mysql. Illegal mix of collations (latin1_general_cs,IMPLICIT) and (latin1_general_ci,IMPLICIT) for operation '=' Any idea on what might be going wrong here? The collation of the table is latin1_general_ci and that of the column in the where clause is latin1_general_cs Thanks!

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  • java mapping for oracle number type

    - by aauser
    I have database with oracle number type. Could there any possible issues with using java Double when updating table with numeric column. Should i switch BigDecimal or Double is approriate type. As I fas as I know, BigDecimal is used when arbitrary precision required, when deal with prices for example. Consider simple API updatePrice(Double d) Should i persist double as is or BigDecimal.valueOf(d) is preferable ?

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