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• What algorithm can calculate the power set of a given set?

  - by ross

  I would like to efficiently generate a unique list of combinations of numbers based on a starting list of numbers. example start list = [1,2,3,4,5] but the algorithm should work for [1,2,3...n] result = [1],[2],[3],[4],[5] [1,2],[1,3],[1,4],[1,5] [1,2,3],[1,2,4],[1,2,5] [1,3,4],[1,3,5],[1,4,5] [2,3],[2,4],[2,5] [2,3,4],[2,3,5] [3,4],[3,5] [3,4,5] [4,5] Note. I don't want duplicate combinations, although I could live with them, eg in the above example I don't really need the combination [1,3,2] because it already present as [1,2,3]

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• Transactional isolation level needed for safely incrementing ids

  - by Knut Arne Vedaa

  I'm writing a small piece of software that is to insert records into a database used by a commercial application. The unique primary keys (ids) in the relevant table(s) are sequential, but does not seem to be set to "auto increment". Thus, I assume, I will have to find the largest id, increment it and use that value for the record I'm inserting. In pseudo-code for brevity: id = select max(id) from some_table id++ insert into some_table values(id, othervalues...) Now, if another thread started the same transaction before the first one finished its insert, you would get two identical ids and a failure when trying to insert the last one. You could check for that failure and retry, but a simpler solution might be setting an isolation level on the transaction. For this, would I need SERIALIZABLE or a lower level? Additionally, is this, generally, a sound way of solving the problem? Are the any other ways of doing it?

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• Complex Join - involving date ranges and sum...

  - by calumbrodie

  I have two tables that I need to join... I want to join table1 and table2 on 'id' - however in table two id is not unique. I only want one value returned for table two, and this value represents the sum of a column called 'total_sold' - within a specified date range (say one month).. SELECT ta.id, tb.total_sold as total_sold_this_week FROM table_a as ta LEFT JOIN table_b as tb ON ta.id=tb.id AND tb.date_sold BETWEEN ADDDATE(NOW(),INTERVAL -3 WEEK) AND NOW() this works but does not SUM the rows - only returning one row for each id... how do I get the sum from table b instead of only one row??? Please criticise if format of question could use more work - I can rewrite and provide sample data if required - this is a trivialised version of a much larger problem. -Thanks

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• Password reset by email without a database table

  - by jpatokal

  The normal flow for resetting a user's password by mail is this: Generate a random string and store it in a database table Email string to user User clicks on link containing string String is validated against database; if it matches, user's pw is reset However, maintaining a table and expiring old strings etc seems like a bit of an unnecessary hassle. Are there any obvious flaws in this alternative approach? Generate a MD5 hash of the user's existing password Email hash string to user User clicks on link containing string String is validated by hashing existing pw again; if it matches, user's pw is reset Note that the user's password is already stored in a hashed and salted form, and I'm just hashing it once more to get a unique but repeatable string. And yes, there is one obvious "flaw": the reset link thus generated will not expire until the user changes their password (clicks the link). I don't really see why this would be a problem though -- if the mailbox is compromised, the user is screwed anyway.

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• SQL multiple primary keys - localization

  - by Max Malmgren

  I am trying to implement some localization in my database. It looks something like this (prefixes only for clarification) tbl-Categories ID Language Name tbl-Articles ID CategoryID Now, in my tbl-Categories, I want to have primary keys spanning ID and language, so that every combination of ID and language is unique. In tbl-Articles I would like a foreign key to reference ID in categories, but not Language, since I do not want to bind an article to a certain language, only category. Of course, I cannot add a foreign key to part of the primary key. I also cannot have the primary key only on the ID of categories, since then there can only be one language. Having no primary keys disables foreign keys altogether, and that is also not a great solution. Do you have any ideas how I can solve this in an elegant fashion? Thanks.

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• Advice on e-commerce logging strategy

  - by yalestar

  I recently inherited an e-commerce app (Java/Struts) that I'm porting to Rails. The thing is, we frequently have to do forensics on orders by poring through the log files, and with the old app's logs (log4j wall of text) it's pretty hard to make sense of the individual orders when several people are placing orders simultaneously. So I'm soliciting advice on a good strategy for logging of these orders, like maybe logging each individual order to its own MongoDB collection based on unique cart ID? Or maybe group them by IP address? Something different entirely? Essentially, what is the best approach for logging of an online store so that it's easy to backtrace each user's interaction with the site?

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• INSERT INTO ... SELECT FROM ... ON DUPLICATE KEY UPDATE

  - by dnagirl

  I'm doing an insert query where most of many columns would need to be updated to the new values if a unique key already existed. It goes something like this: INSERT INTO lee(exp_id, created_by, location, animal, starttime, endtime, entct, inact, inadur, inadist, smlct, smldur, smldist, larct, lardur, lardist, emptyct, emptydur) SELECT id, uid, t.location, t.animal, t.starttime, t.endtime, t.entct, t.inact, t.inadur, t.inadist, t.smlct, t.smldur, t.smldist, t.larct, t.lardur, t.lardist, t.emptyct, t.emptydur FROM tmp t WHERE uid=x ON DUPLICATE KEY UPDATE ...; //update all fields to values from SELECT, except for exp_id, created_by, location, animal, starttime, endtime I'm not sure what the syntax for the UPDATE clause should be. How do I refer to the current row from the SELECT clause?

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• Identifying CDs

  - by Chris

  I'd like to be able to determine what music album CD is in a CD drive. For example, if someone claims that the CD in their drive is Eminem - The Eminem Show, I would like to be able to verify that the CD is indeed The Eminem Show. Any ideas? I've applied for a Gracenote developer license, but they won't get back to me for five days. Also, how does this work? Is there some GUID or other unique identifier that music discs are encoded with? Lastly, might this be possible with data CDs, like, say, the Diablo II install Disc 1? If so, any directions you can point me in, for accomplishing this?

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• What is the best method to write user "log files" of an android application to a file in a remote server or a table in a remote database?

  - by Samitha Chathuranga

  I am creating a multi user android application and it is connected to a php web service in a remote server and to a remote database via that web service. I want to keep a track of all the important activities done by the users. For an example if a user logged in to the app and changed his profile details and then logged out, a brief description of what he has done should be recorded(with time) somewhere. So then the admin of the system can see what the users are doing. So I think it is better to use log cat files and then flush all those data to a unique file in the server or a table in the database, when the user logs out or exists from his account. If it is appropriate How to do it?

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• adding DATE_SUB to query to return range of values in mysql

  - by ian

  Here is my original query: $query = mysql_query("SELECT s.*, UNIX_TIMESTAMP(`date`) AS `date`, f.userid as favoritehash FROM songs s LEFT JOIN favorites f ON f.favorite = s.id AND f.userid = '$userhash' ORDER BY s.date DESC"); This returns all the songs in my DB and then joins data from my favorites table so I can display wich items a return visitors has clicked as favorites or not. Visitors are recognized by a unique has storred in a cookie and in the favorites table. I need to alter this query so that I can get just the last months worth of songs. Below is my attempt at adding DATE_SUB to my query: $query = mysql_query("SELECT s.*, UNIX_TIMESTAMP(`date`) AS `date`, f.userid as favoritehash FROM songs s WHERE `date` >= DATE_SUB( NOW( ) , INTERVAL 1 MONTH ) LEFT JOIN favorites f ON f.favorite = s.id AND f.userid = '$userhash' ORDER BY s.date DESC"); Suggestions?

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• How to perform COUNT() or COUNT(*)

  - by ninumedia

  I have a list of tags in a database. Ex: villan hero spiderman superman superman I wanted to obtain a sorted list of the tag names in ascending order and the number of times the unique tag appeared in the database. I wrote this code: Ex: SELECT hashtag.tag_name, COUNT( * ) AS number FROM hashtag GROUP BY hashtag.tag_name ORDER BY hashtag.tag_name ASC This yields the correct result: hero - 1 spiderman - 1 superman - 2 villan - 1 How can I obtain the full COUNT of this entire list. The answer should be 4 in this case because there are naturally 4 rows. I can't seem to get a correct COUNT() without the statement failing. Thanks so much for the help! :)

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• How can I pipe a large amount of data as a runtime argument?

  - by Zombies

  Running an executable JAR on a linux platform here. The program it self works on a somewhat large amount of data, basically a list of URLs... could be up to 2k. Currently I get this from a simple DB call. But I was thinking that instead of creating a new mode and writing SQL to get a new result set and having to redploy everytime, I could just make the program more robust by passing in the result set (the list of URLs) that need to be worked on... so, within a linux environment, is there a pain-free/simple way to get the result set and pass it in dynamically? I know file i/o is one, but it doesn't seem to be effecient because each file has to be named, as well more logic to handle grabbing the correct file, creating a file with a unique name, etc.

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• Random Number on SQL without using NewID()

  - by Angel Escobedo

  Hello I want to generate a Unique Random number with out using the follow statement : Convert(int, (CHECKSUM(NEWID()))*100000) AS [ITEM] Cause when I use joins clauses on "from" it generates double registers by using NEWID() Im using SQL Server 2000 *PD : When I use Rand() it probably repeat on probability 1 of 100000000 but this is so criticall so it have to be 0% of probability to repeat a random value generated My Query with NewID() and result on SELECT statement is duplicated (x2) My QUery without NewID() and using Rand() on SELECT statement is single (x1) but the probability of repeat the random value generated is uncertainly but exists! Thanks!

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• gethostbyname in C

  - by Matic

  I don't know how to write applications in C, but I need a tiny program that does: lh = gethostbyname("localhost"); output = lh->h_name; output variable is to be printed. The above code is used in PHP MongoDB database driver to get the hostname of the computer (hostname is part of an input to generate an unique ID). I'm skeptical that this will return the hostname, so I'd like some proof. Any code examples would be most helpful. Happy day, Matic

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• Detecting if a browser is using Private Browsing mode

  - by Steve

  I'm building an extranet for a company paranoid about security. They want to make sure that (among other things) their users are browsing the site with the Private Browsing mode switched on in their web browser so that no cookies or history is kept. I found only this http://jeremiahgrossman.blogspot.com/2009/03/detecting-private-browsing-mode.html and http://serverfault.com/questions/18966/force-safari-to-operate-in-private-mode-and-detect-that-state-from-a-webserver The ideal solution would use no or minimal javascript. Would attempting to set a unique cookie work for all browsers and platforms? Anyone done this before? thanks!

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• how do I get the index of a firing context menu item

  - by fishhead

  I would like to determine the index of the selected context menu item. But I don't want to build a unique eventhandler for each menu item...how do I do this MenuItem mi= StatusContextMenu.MenuItems.Add("Set option"); mi.MenuItems.Add(0,new MenuItem("Set to A", new EventHandler(SetLetter)) ); mi.MenuItems.Add(1,new MenuItem("Set to B", new EventHandler(SetLetter))); mi.MenuItems.Add(2,new MenuItem("Set to C ", new EventHandler(SetLetter))); mi.MenuItems.Add(3,new MenuItem("Set to D", new EventHandler(SetLetter))); private void SetLetter(object sender, System.EventArgs e) { index = ???? } C#, .net 2.0

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• Selecting an element based on class and id in jquery - similar to an SQL query

  - by Ankur

  I want to select an element based on it's class and id, I have seen many similar questions on SO. Most of the users ask why would you ever want to do this - which is not very helpful, trust me, this is the simplest way of solving my UI problem, and it's a very unique and intereting UI related to making generic triple stores easily browsable. At the moment the jQUery looks like this: $(document).ready( function() { $(".predicates").hide(); $(".objects").hide(); $("#subject").click( function() { $("#predId, .predicates").toggle(); // this line } ); $("#predId").click( function() { $("#objId, .objects").toggle(); } ); }); The line I am really interested in is this $("#predId, .predicates").toggle(); how do I get the select to ensure that both #predId and .predicates are true, at the moment it seems that only one or the other has to be true. Is there something like an && operator?

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• django m2m how can i get m2m table elements in a view

  - by dana

  i have a model using m2m feature: class Classroom(models.Model): user = models.ForeignKey(User, related_name = 'classroom_creator') classname = models.CharField(max_length=140, unique = True) date = models.DateTimeField(auto_now=True) open_class = models.BooleanField(default=True) members = models.ManyToManyField(User,related_name="list of invited members", through = 'Membership') and i want to take all members of one class in a view and display them using the template system. In the view, i'm trying to take all the members from a classroom like that: def inside_classroom(request,classname): try: theclass = Classroom.objects.get(classname = classname) members = Members.objects.all() etc but it doesn't work,(though the db_table is named Classroom_Members) i guess i have to use another query for getting all the members from the classroom classname. also, i want to verify if the request.user is a member using (if request.user in members) how can i het those members? Thanks in advance!

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• Quickly retrieve the subset of properties used in a huge collection in C#

  - by ccornet

  I have a huge Collection (which I can cast as an enumerable using OfType<()) of objects. Each of these objects has a Category property, which is drawn from a list somewhere else in the application. This Collection can reach sizes of hundreds of items, but it is possible that only, say, 6/30 of the possible Categories are actually used. What is the fastest method to find these 6 Categories? The size of the huge Collection discourages me from just iterating across the entire thing and returning all unique values, so is there a faster method of accomplishing this? Ideally I'd collect the categories into a List.

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• Hibernate: same generated value in two properties

  - by Markos Fragkakis

  Hi, I have an entity A with fields: aId (the system id) bId I want the first to be generated: @Id @Column(name = "PRODUCT_ID", unique = true, nullable = false, precision = 12, scale = 0) @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "PROD_GEN") @BusinessKey public Long getAId() { return this.aId; } I want the bId to be initially exactly as the aId. One approach is to insert the entity, then get the aId generated by the DB (2nd query) and then update the entity, setting the bId to be equal to aId (3rd query). Is there a way to get the bId to get the same generated value as aId? Note that afterwards, I want to be able to update bId from my gui. If the solution is JPA, even better.

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• mongoid with rails - Database should be a Mongo::DB, not NilClass"

  - by Adam T

  Greetings I am trying to get Mongoid to work with my Rails app and I am getting an error: "Mongoid::Errors::InvalidDatabase in 'Shipment bol should be unique' Database should be a Mongo::DB, not NilClass" I have created the mongoid.yml file in my config directory and have mongodb running as a daemon. The config file is like so: defaults: &defaults host: localhost development: <<: *defaults database: ship-it-development test: <<: *defaults database: ship-it-test production: <<: *defaults host: <%= ENV['MONGOID_HOST'] % port: <%= ENV['MONGOID_PORT'] % database: <%= ENV['MONGOID_DATABASE'] % All of my specs fail with the above error. I am using rails 2.3.8. Anyone have ideas?

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• Force 'Replace Into' to use a certain index

  - by Bobby

  I have a MySQL (5.0) table with 3 rows which are considered a combined Unique Index: CREATE TABLE `test`.`table_a` ( `Id` int(11) NOT NULL AUTO_INCREMENT, `field1` varchar(5) COLLATE latin1_swedish_ci NOT NULL DEFAULT '', `field2` varchar(5) COLLATE latin1_swedish_ci NOT NULL DEFAULT '', `field3` varchar(5) COLLATE latin1_swedish_ci NOT NULL DEFAULT '', PRIMARY KEY (`Id`), INDEX `IdxUnqiue` (`field1`(5),`field2`(5),`field3`(5)) ) ENGINE=MyISAM; This table should be filled with a REPLACE INTO query: REPLACE INTO table_a ( Field1, Field2, Field3 ) VALUES ( "Test1", "Test2", "Test3" ) The behavior I'd like to see is that this query always overrides the previous inserted row, because IdxUnique is...ahm, triggered. But unfortunately, there's still the primary index which seems to kick in and always inserts a new row. What I get: Query was executed 3 times: +---Id---+---Field1---+---Field2---+---Field3---+ | 1 | Test1 | Test2 | Test2 | | 2 | Test1 | Test2 | Test2 | | 3 | Test1 | Test2 | Test2 | +--------+------------+------------+------------+ What I want: Query was executed 3 times: +---Id---+---Field1---+---Field2---+---Field3---+ | 3 | Test1 | Test2 | Test2 | +--------+------------+------------+------------+ So, can I tell REPLACE INTO to use just a certain Index or to consider one 'more inportant' then another?

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• Oracle - Is there any effects of not having a primary key on a table ?

  - by Sathya

  We use sequence numbers for primary keys on the tables. There are some tables where we dont really use the primary key for any querying purpose. But, we have Indexes on other columns. These are non-unique indexes. The queries use these non-primary key columns in the WHERE conditions. So, I dont really see any benefit of having a primary key on such tables. My experience with SQL 2000 was that, it used to replicate tables which had some primary key. Otherwise it would not. I am using Oracle 10gR2. I would like to know if there are any such side-effects of having tables that dont have primary key.

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• List of all index & index columns in SQL Server DB

  - by Anton Gogolev

  How do I get a list of all index & index columns in SQL Server 2005+? The closest I could get is: select s.name, t.name, i.name, c.name from sys.tables t inner join sys.schemas s on t.schema_id = s.schema_id inner join sys.indexes i on i.object_id = t.object_id inner join sys.index_columns ic on ic.object_id = t.object_id inner join sys.columns c on c.object_id = t.object_id and ic.column_id = c.column_id where i.index_id > 0 and i.type in (1, 2) -- clustered & nonclustered only and i.is_primary_key = 0 -- do not include PK indexes and i.is_unique_constraint = 0 -- do not include UQ and i.is_disabled = 0 and i.is_hypothetical = 0 and ic.key_ordinal > 0 order by ic.key_ordinal which is not exactly what I want. What I want is to list all user-defined indexes (which means no indexes which support unique constraints & primary keys) with all columns (ordered by how do they apper in index definition) plus as much metadata as possible.

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• Are concurrency issues possible when using the WCF Service Behavoir attribute set to ConcurrencyMode

  - by Brandon Linton

  We have a WCF service that makes a good deal of transactional NHibernate calls. Occasionally we were seeing SQL timeouts, even though the calls were updating different rows and the tables were set to row level locking. After digging into the logs, it looks like different threads were entering the same point in the code (our transaction using block), and an update was hanging on commit. It didn't make sense, though, because we believed that the following service class attribute was forcing a unique execution thread per service call: [ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple, InstanceContextMode = InstanceContextMode.PerCall)] We recently changed the concurrency mode to ConcurrencyMode.Single and haven't yet run into any issues, but the bug was very difficult to reproduce (if anyone has any thoughts on flushing a bug like that out, let me know!). Anyway, that all brings me to my question: shouldn't an InstanceContextMode of PerCall enforce thread-safety within the service, even if the ConcurrencyMode is set to multiple? How would it be possible for two calls to be serviced by the same service instance? Thanks!

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