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• How can an SQL query return data from multiple tables

  - by Fluffeh

  I would like to know how to get data from multiple tables in my database, what types of methods are there to do this, what are joins and unions and how are they different from one another? When should I use each one compared to the others? I am planning to use this in my (for example - PHP) application, but don't want to run multiple queries against the database, what options do I have to get data from multiple tables in a single query? Note: I am writing this as I would like to be able to link to a well written guide on the numerous questions that I constantly come across in the PHP queue, so I can link to this for further detail when I post an answer. The answers cover off the following: Part 1 - Joins and Unions Part 2 - Subqueries Part 3 - Tricks and Efficient Code

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• execute javascsript for loop alternately forward and backward

  - by Stiff Mittens

  What I'm trying to do is iterate through an array in chunks, alternating the direction of iteration from chunk to chunk. Confused? So am I. For example, if I want to loop thru an array with 25 elements, but I want to do it in this order: 0, 1, 2, 3, 4, 9, 8, 7, 6, 5, 10, 11, 12, 13, 14, 19, 18, 17, 16, 15, 20, 21, 22, 23, 24, what would be the most efficient way of doing this? I'm looking for something scalable because the array I'm working with now is actually 225 elements and I want to traverse it in 15 element chunks, but that may change at some point. So far, the only method I've figured out that actually works is to hard-wire the iteration order into a second array and then iterate through that in the normal way to get the indices for the original array. But that sucks. Any help would be greatly appreciated.

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• How to Implement chat application for Android?

  - by user396530

  I am working on a chat application for android. This chat application is for sending messages from one device to another using internet(GPRS,3G,etc) from this application. please tell me a way to implement this. I thought using web services is more data(internet) consuming and less efficient.is this right? I worked on server and client sockets. I ran both server and client classes in single device and messages can be transfered from client socket to server socket and vice versa.Now i want to message between two devices using server socket on web server and how to connect to server from Android Devices. please help me thank you very much.

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• Need advice on cron job'ing a very large process

  - by Arms

  I have a PHP script that grabs data from an external service and saves data to my database. I need this script to run once every minute for every user in the system (of which I expect to be thousands). My question is, what's the most efficient way to run this per user, per minute? At first I thought I would have a function that grabs all the user Ids from my database, iterate over the ids and perform the task for each one, but I think that as the number of users grow, this will take longer, and no longer fall within 1 minute intervals. Perhaps I should queue the user Ids, and perform the task individually for each one? In which case, I'm actually unsure of how to proceed. Thanks in advance for any advice.

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• Help with algorithm to determine phrase occurrence in string in PHP

  - by Will M.

  I have an array of phrases (max 2 words) like $words = array('barack obama', 'chicago', 'united states'); and then I have a string like: $sentence = "Barack Obama is from Chicago. Barack Obama's favorite food it pizza."; I want to find/create an efficient algorithm that would return the number of occurrences of the words in the array $words in the string $sentence. In this case it would be: 'barack obama' => 2 'chicago' => 0 How can I built this?

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• Overhead of serving pages - JSPs vs. PHP vs. ASPXs vs. C

  - by John Shedletsky

  I am interested in writing my own internet ad server. I want to serve billions of impressions with as little hardware possible. Which server-side technologies are best suited for this task? I am asking about the relative overhead of serving my ad pages as either pages rendered by PHP, or Java, or .net, or coding Http responses directly in C and writing some multi-socket IO monster to serve requests (I assume this one wins, but if my assumption is wrong, that would actually be most interesting). Obviously all the most efficient optimizations are done at the algorithm level, but I figure there has got to be some speed differences at the end of the day that makes one method of serving ads better than another. How much overhead does something like apache or IIS introduce? There's got to be a ton of extra junk in there I don't need. At some point I guess this is more a question of which platform/language combo is best suited - please excuse the in-adroitly posed question, hopefully you understand what I am trying to get at.

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• How to find and update the next record in a linking table

  - by someoneinomaha

  I have a classifieds system I'm working on. People are able to add photos to a classified, but I only display one when displaying the list of classifieds. To do that, I have a linking table between classifieds and photos that has a "is_main" boolean field. When someone deletes one of their classified photos, I want to: 1) See if there is more than that photo tied to the classified. 2) If there is, update the next photo and set that "is_main" field to TRUE. Just trying to find out the most efficient way to do this.

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• Is this the best way to get the size of a Paper via CUPS?

  - by Lawrence Johnston

  I've got a CUPS job control file. I need to get the actual dimensions of the print that was made. This is the way I've figure out to do it: Get the paper name using the media attribute. (e.g. Letter) Get the printer uri from the printer-uri attribute. (e.g. ipp://localhost/printers/MyPrinter) Get the printer name by passing the printer uri to the IPP_GET_PRINTER_ATTRIBUTES operation and getting the printer-name attribute. (e.g. MyPrinter) Get the path to the PPD passing the printer name to the the cupsGetPPD method. Open the PPD passing the path to the PPD to the ppdOpenFile method. Get the paper size by passing the PPD and paper name to the ppdPageSize method. This will work, but it seems a bit roundabout. Is there a more efficient way of getting what I need?

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• How to solve recursion relations in mathematica efficiently?

  - by Qiang Li

  I have a recursion to solve for. f(m,n)=Sum[f[m - 1, n - 1 - i] + f[m - 3, n - 5 - i], {i, 2, n - 2*m + 2}] + f[m - 1, n - 3] + f[m - 3, n - 7] f(0,n)=1, f(1,n)=n However, the following mma code is very inefficient f[m_, n_] := Module[{}, If[m < 0, Return[0];]; If[m == 0, Return[1];]; If[m == 1, Return[n];]; Return[Sum[f[m - 1, n - 1 - i] + f[m - 3, n - 5 - i], {i, 2, n - 2*m + 2}] + f[m - 1, n - 3] + f[m - 3, n - 7]];] It takes unbearably long to compute f[40,20]. Could anyone please suggest an efficient way of doing this? Many thanks!

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• Custom deleters for std::shared_ptrs

  - by Kristian D'Amato

  Is it possible to use a custom deleter after creating a std::shared_ptr without using new? My problem is that object creation is handled by a factory class and its constructors & destructors are protected, which gives a compile error, and I don't want to use new because of its drawbacks. To elaborate: I prefer to create shared pointers like this, which doesn't let you set a custom deleter (I think): auto sp1 = make_shared<Song>(L"The Beatles", L"Im Happy Just to Dance With You"); Or I can create them like this, which does let met set a deleter through an argument: auto sp2(new Song, MyDeleterFunc); But the second one uses new, which AFAIK isn't as efficient as the top sort of allocation. Maybe this is clearer: is it possible to get the benefits of make_shared<> as well as a custom deleter? Would that mean having to write an allocator?

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• Find all numbers that appear in each of a set of lists

  - by Ankur

  I have several ArrayLists of Integer objects, stored in a HashMap. I want to get a list (ArrayList) of all the numbers (Integer objects) that appear in each list. My thinking so far is: Iterate through each ArrayList and put all the values into a HashSet This will give us a "listing" of all the values in the lists, but only once Iterate through the HashSet 2.1 With each iteration perform ArrayList.contains() 2.2 If none of the ArrayLists return false for the operation add the number to a "master list" which contains all the final values. If you can come up with something faster or more efficient, funny thing is as I wrote this I came up with a reasonably good solution. But I'll still post it just in case it is useful for someone else. But of course if you have a better way please do let me know.

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• How to save an order (permutation) in an sql db

  - by Bendlas

  I have a tree structure in an sql table like so: CREATE TABLE containers ( container_id serial NOT NULL PRIMARY KEY, parent integer REFERENCES containers (container_id)) Now i want to define an ordering between nodes with the same parent. I Have thought of adding a node_index column, to ORDER BY, but that seem suboptimal, since that involves modifying the index of a lot of nodes when modifying the stucture. That could include adding, removing, reordering or moving nodes from some subtree to another. Is there a sql datatype for an ordered sequence, or an efficient way to emulate one? Doesn't need to be fully standard sql, I just need a solution for mssql and hopefully postgresql EDIT To make it clear, the ordering is arbitrary. Actually, the user will be able to drag'n'drop tree nodes in the GUI

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• linq to sql update data

  - by pranay

  can i update my employee record as given in below function or i have to make query of employee collection first and than i update data public int updateEmployee(App3_EMPLOYEE employee) { DBContextDataContext db = new DBContextDataContext(); db.App3_EMPLOYEEs.Attach(employee); db.SubmitChanges(); return employee.PKEY; } or i have to do this public int updateEmployee(App3_EMPLOYEE employee) { DBContextDataContext db = new DBContextDataContext(); App3_EMPLOYEE emp = db.App3_EMPLOYEEs.Single(e => e.PKEY == employee.PKEY); db.App3_EMPLOYEEs.Attach(employee,emp); db.SubmitChanges(); return employee.PKEY; } But i dont want to use second option is there any efficient way to update data

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• Preventing a security breach

  - by Wiz

  I am creating a website where you "post", and the form content is saved in a MySql database, and upon loading the page, is retrieved, similar to facebook. I construct all the posts and insert raw html into a template. The thing is, as I was testing, I noticed that I could write javascript or other HTML into the form and submit it, and upon reloading, the html or JS would treated as source code, not a post. I figured that some simple encoding would do the trick, but using is not working. Is there an efficient way to prevent this type of security hole?

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• Efficiently Reshaping/Reordering Numpy Array to Properly Ordered Tiles (Image)

  - by Phelix

  I would like to be able to somehow reorder a numpy array for efficient processing of tiles. what I got: >>> A = np.array([[1,2],[3,4]]).repeat(2,0).repeat(2,1) >>> A # image like array array([[[1, 1, 2, 2], [1, 1, 2, 2]], [[3, 3, 4, 4], [3, 3, 4, 4]]]) >>> A.reshape(2,2,4) array([[[1, 1, 2, 2], [1, 1, 2, 2]], [[3, 3, 4, 4], [3, 3, 4, 4]]]) what I want: X >>> X array([[[1, 1, 1, 1], [2, 2, 2, 2]], [[3, 3, 3, 3], [4, 4, 4, 4]]]) to be able to do something like: >>> X[X.sum(2)>12] -= 1 >>> X array([[[1, 1, 1, 1], [2, 2, 2, 2]], [[3, 3, 3, 3], [3, 3, 3, 3]]]) Is this possible without a slow python loop? Bonus: Conversion back from X to A Edit: How can I get X from A?

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• More intelligent Context Menus in Visual Studio 2008 Database Projects

  - by Mike C.

  I'm using a SQL Server 2008 Database Project, and I'm finding it's very cumbersome to add certain items. For example, if I right click on Tables = Keys and navigate to the Add item, I'd expect to see the different kinds of keys that I can add. Instead, I see the stock items that don't apply, and I'm forced to navigate the Add New Item screen to find Primary Key. Is there a way to make this more efficient? I have Resharper; can I leverage templates somehow (kind of a noob with these). Thanks!

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• Generic Database table design

  - by Gazeth

  Just trying to figure out the best way to design my table for the following scenario: I have several areas in my system (documents, projects, groups and clients) and each of these can have comments logged against them. My question is should I have one table like this: CommentID DocumentID ProjectID GroupID ClientID etc Where only one of the ids will have data and the rest will be NULL or should I have a seperate CommentType table and have my comments table like this: CommentID CommentTypeID ResourceID (this being the id of the project/doc/client) etc My thoughts are that option 2 would be more efficient from an indexing point of view?

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• Most optimal way to detect if black (or any color pixels) exist in an image file?

  - by Zando

  What's the best and most flexible algorithm to detect any black (or colored pixel) in a given image file? Say I'm given an image file that could, say, have a blue background. And any non blue pixel, including a white pixel, is counted as a "mark". The function returns true if there are X number of pixels that deviate from each other at a certain threshold. I thought it'd be fastest to just simply iterate through every pixel and see if its color matches the last. But if it's the case that pixel (0,0) is deviant, and every other pixel is the same color (and I want to allow at least a couple deviated pixels before considering an image to be "marked), this won't work or be terribly efficient.

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• enumerate all combinations in c++

  - by BCS

  My question is similar to this combinations question but in my case I have N (N 4) small sets (1-2 items per set for now might go to 3 maybe 4) and want to generate each combination of one item from each set. The current solution looks somethinging along the lines of this for(T:: iterator a = setA.begin(); a != setA.end(); ++a) for(T:: iterator b = setB.begin(); b != setB.end(); ++b) for(T:: iterator c = setC.begin(); c != setC.end(); ++c) for(T:: iterator d = setD.begin(); d != setD.end(); ++d) for(T:: iterator e = setE.begin(); e != setE.end(); ++e) something(*a,*b,*c,*d,*e); Simple, effective, probably reasonably efficient, but ugly and not very extensible. Does anyone know of a better/cleaner way to do this?

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• Accessing two sides of a user-user relationship in rails

  - by Lowgain

  Basically, I have a users model in my rails app, and a fanship model, to facilitate the ability for users to become 'fans' of each other. In my user model, I have: has_many :fanships has_many :fanofs, :through => :fanships In my fanship model, I have: belongs_to :user belongs_to :fanof, :class_name => "User", :foreign_key => "fanof_id" My fanship table basically consists of :id, :user_id and :fanof_id. This all works fine, and I can see what users a specific user is a fan of like: <% @user.fanofs.each do |fan| %> #things <% end %> My question is, how can I get a list of the users that are a fan of this specific user? I'd like it if I could just have something like @user.fans, but if that isn't possible what is the most efficient way of going about this? Thanks!

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• How to correctly migrate urls from custom asp.net solution to Wordpress?

  - by Marek

  I have a web site built using asp.net with ugly URLs like /DisplayContent.aspx?id=789564. I know how to migrate the database, but the Wordpress urls will be (naturally) different. Can I simply write some mapping or do I have to include a rewrite rule for each subpage (300 pages) in .htaccess? Should I provide a rewrite rule for each existing page that would transform a full old url to the known new url, like for example: /DisplayContent.aspx?id=789798 -> /2010-5-10/Title-Of-The-Post Even if I manage to migrate the URLs, the structure of the HTML for the new content will naturally be different. How does this affect SEO? Should I run asp.net and wordpress side by side and issue the redirects from the asp.net application? What is the most efficient solution to this kind of migration of URLs without doing PHP programming?

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• Generating Fibonacci series in F#

  - by photo_tom

  I'm just starting to learn F# using VS2010 and below is my first attempt at generating the Fibonacci series. What I'm trying to do is to build a list of all numbers less than 400. let fabList = let l = [1;2;] let mutable a = 1 let mutable b = 2 while l.Tail < 400 do let c = a + b l.Add(c) let a = b let b = c My first problem is that on the last statement, I'm getting an error message "Incomplete structured construct at or before this point in expression" on the last line. I don't understand what I'm doing wrong here. While this seems to be an obvious way to build the list in a fairly efficient way (from a c++/C# programmer), from what little I know of f#, this doesn't seem to feel to be the right way to do the program. Am I correct in this feeling?

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• hunting maven dependencies

  - by Tom

  I want to start using maven in code I distribute but I can't find an efficient way to work with dependencies. Every new dependency takes me far too long to add. As a simple example, I need to add Tomcat for compilation. Do I really have to manually trawl the repo in my browser to find the group-id, artifact-id and version number? In every case it seems easier to find the non-maven downloads. I hope I've missed something obvious.

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• MySQL "IS IN" equivalent?

  - by nute

  A while ago I worked on a MS-SQL project and I remember a "IS IN" thing. I tried it on a MySQL project and it did not work. Is there an equivalent? Workaround? Here is the full query I am trying to run: SELECT * FROM product_product, product_viewhistory, product_xref WHERE ( (product_viewhistory.productId = product_xref.product_id_1 AND product_xref.product_id_2 = product_product.id) OR (product_viewhistory.productId = product_xref.product_id_2 AND product_xref.product_id_1 = product_product.id) ) AND product_product.id IS IN (SELECT DISTINCT pvh.productId FROM product_viewhistory AS pvh WHERE pvh.cookieId = :cookieId ORDER BY pvh.viewTime DESC LIMIT 10) AND product_viewhistory.cookieId = :cookieId AND product_product.outofstock='N' ORDER BY product_xref.hits DESC LIMIT 10 It's pretty big ... but the part I am interested in is: AND product_product.id IS IN (SELECT DISTINCT pvh.productId FROM product_viewhistory AS pvh WHERE pvh.cookieId = :cookieId ORDER BY pvh.viewTime DESC LIMIT 10) Which basically says I want the products to be in the "top 10" of that sub-query. How would you achieve that with MySQL (while trying to be efficient)?

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• delete all records except the id I have in a python list

  - by jay_t

  Hi all, I want to delete all records in a mysql db except the record id's I have in a list. The length of that list can vary and could easily contain 2000+ id's, ... Currently I convert my list to a string so it fits in something like this: cursor.execute("""delete from table where id not in (%s)""",(list)) Which doesn't feel right and I have no idea how long list is allowed to be, .... What's the most efficient way of doing this from python? Altering the structure of table with an extra field to mark/unmark records for deletion would be great but not an option. Having a dedicated table storing the id's would indeed be helpful then this can just be done through a sql query... but I would really like to avoid these options if possible. Thanks,

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