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Search found 1908 results on 77 pages for 'relational operators'.

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• Conditional Join - join 1 tables 2 ways

  - by Jon H

  I have a set of (not very well normalised or relational) tables named PLAN, GROUP, PRODUCT CLIENT Most have linkage i.e. PLAN - CLIENT on clno GROUP to PRODUCT on PRODCD However, the linkage between PLAN and GROUP is tricky. A plan has 2 field of interest GRPNO and PRODCD. What I want to do is if GRPNO != 0 then join GROUP on GRPNO. However if GRPNO = 0 then I want to join GROUP on PRODCD. The frustrating thing is that the fileds I want to return in my queries are the same across the board I just need to be able to vary the join, or join the same table twice. The best I can come up with is 2 queries and merge them using datasets, or possibly using a union. Is there a nifty way to do this in one select? I should point out I am access Foxpro over ODBC to do this. Thank you!

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• math syntax checker written in python

  - by neurino

  All I need is to check, using python, if a string is a valid math expression or not. For simplicity let's say I just need + - * / operators (+ - as unary too) with numbers and nested parenthesis. I add also simple variable names for completeness. So I can test this way: test("-3 * (2 + 1)") #valid test("-3 * ") #NOT valid test("v1 + v2") #valid test("v2 - 2v") #NOT valid ("2v" not a valid variable name) I tried pyparsing but just trying the example: "simple algebraic expression parser, that performs +,-,*,/ and ^ arithmetic operations" I get passed invalid code and also trying to fix it I always get wrong syntaxes being parsed without raising Exceptions just try: >>>test('9', 9) 9 qwerty = 9.0 ['9'] => ['9'] >>>test('9 qwerty', 9) 9 qwerty = 9.0 ['9'] => ['9'] both test pass... o_O Any advice?

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• Extracting Demographic and Contact Information from unstructured text files

  - by jn29098

  I am looking to extract specific items out of a large pool of unstructured documents. These documents could be 1-5 pages of text formatted in various ways by the user, but in most cases would contain at least: Name Address (physical) Email Address Phone number website URL I'm looking for a semantic parser that can attempt to extract these elements from the documents so that I can load that information into a relational database and work with these records as contacts. Other services I've looked for, while valuable for other purposes, do not address this specific need. Alchemy API Open Calais Saplo Any thoughts, suggestions or leads?

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• Understanding pattern matching with cons operator

  - by Mathias

  In "Programming F#" I came across a pattern-matching like this one (I simplified a bit): let rec len list = match list with | [] -> 0 | [_] -> 1 | head :: tail -> 1 + len tail;; Practically, I understand that the last match recognizes the head and tail of the list. Conceptually, I don't get why it works. As far as I understand, :: is the cons operator, which appends a value in head position of a list, but it doesn't look to me like it is being used as an operator here. Should I understand this as a "special syntax" for lists, where :: is interpreted as an operator or a "match pattern" depending on context? Or can the same idea be extended for types other than lists, with other operators?

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• Can you define <=> in Ruby and then have ==, >, <, >=, and <= defined automatically?

  - by jeremy Ruten

  Here's part of my Note class: class Note attr_accessor :semitones, :letter, :accidental def initialize(semitones, letter, accidental = :n) @semitones, @letter, @accidental = semitones, letter, accidental end def <=>(other) @semitones <=> other.semitones end def ==(other) @semitones == other.semitones end def >(other) @semitones > other.semitones end def <(other) @semitones < other.semitones end end It seems to me like there should be a module that I could include that could give me my equality and comparison operators based on my <=> method. Is there one? I'm guessing a lot of people run into this kind of problem. How do you usually solve it? (How do you make it DRY?)

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• NoSql Crash Course/Tutorial

  - by Chris Thompson

  Hi all, I've seen NoSQL pop up quite a bit on SO and I have a solid understanding of why you would use it (from here, Wikipedia, etc). This could be due to the lack of concrete and uniform definition of what it is (more of a paradigm than concrete implementation), but I'm struggling to wrap my head around how I would go about designing a system that would use it or how I would implement it in my system. I'm really stuck in a relational-db mindset thinking of things in terms of tables and joins... At any rate, does anybody know of a crash course/tutorial on a system that would use it (kind of a "hello world" for a NoSQL-based system) or a tutorial that takes an existing "Hello World" app based on SQL and converts it to NoSQL (not necessarily in code, but just a high-level explanation). I see this having one solid answer, but if you guys feel like it should be community wiki, I'll be happy to change it. Thanks! Chris

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• VB.NET overloading array access?

  - by Wayne Werner

  Hi, Is it possible to overload the array/dict access operators in VB.net? For example, you can state something like: Dim mydict As New Hashtable() mydict.add("Cool guy", "Overloading is dangerous!") mydict("Cool guy") = "Overloading is cool!" And that works just fine. But what I would like to do is be able to say: mydict("Cool guy") = "3" and then have 3 automagically converted to the Integer 3. I mean, sure I can have a private member mydict.coolguy and have setCoolguy() and getCoolguy() methods, but I would prefer to be able to write it the former way if at all possible. Thanks

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• In Ruby, can the coerce() method know what operator it is that requires the help to coerce?

  - by Jian Lin

  In Ruby, it seems that a lot of coerce() help can be done by def coerce(something) [self, something] end that's is, when 3 + rational is needed, Fixnum 3 doesn't know how to handle adding a Rational, so it asks Rational#coerce for help by calling rational.coerce(3), and this coerce instance method will tell the caller: # I know how to handle rational + something, so I will return you the following: [self, something] # so that now you can invoke + on me, and I will deal with Fixnum to get an answer So what if most operators can use this method, but not when it is (a - b) != (b - a) situation? Can coerce() know which operator it is, and just handle those special cases, while just using the simple [self, something] to handle all the other cases where (a op b) == (b op a) ? (op is the operator).

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• 'e-Commerce' scalable database model

  - by Ruben Trancoso

  I would like to understand database scalability so I've just heard a talk about Habits of Highly Scalable Web Applications http://techportal.ibuildings.com/2010/03/02/habits-of-highly-scalable-web-applications/ On it, the presenter mainly talk about relational database scalability. I also have read something about MapReduce and Column oriented tables, big tables, hypertable etc... trying to understand which are the most up to date methods to scale web application data. But the second group, to me, is being hard to understand where it fits. It serves as transactional, reliable data store? or not, its just for large access and processing and to handle fine graned operations we will ever need to rely on RDBMSs? Could someone give a comprehensive landscape for those new technologies and how to use it?

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• What database works well with 200+GB of data?

  - by taw

  I've been using mysql (with innodb; on Amazon rds) because it's sort of universal default, but it's been ridiculously under-performing, and tweaking it only delays the inevitable. The data is mostly relatively short (<1kB of bytes each) blobs information about 100Ms of urls. There is (or should be, mysql cannot seem to handle it) very high amount of insert / update / retrieve but few complex queries - not that complex queries wouldn't be useful, but because mysql is so slow that it's far faster to get the data out, process it locally, and cache the results somewhere. I can keep tweaking mysql and throwing more hardware at it, but it seems increasingly futile. So what are the options? SQL/relational model/etc. optional - anything will do as long as it's fast, networked, and language-independent.

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• Haskell: What is the difference between $ (dollar) and $! (dollar exclamation point)

  - by Jelle Fresen

  Can anybody explain the difference in Haskell between the operators ($) and ($!) (dollar sign vs dollar sign exclamation point)? I haven't seen the use of $! anywhere so far, but while browsing through the Haskell reference on www.zvon.org, I noticed its existence and that it has the exact same definition as $. When trying some simple statements in a Haskell interpreter (ghci), I couldn't find any difference, nor could I find any reference to the operator in the top listed tutorials when googling for haskell tutorial. So, just out of curiosity, what is the difference, if at all?

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• How can I have a serializable struct that wraps it's self as an int32 implicitly? in C#?

  - by firoso

  Long story short, I have a struct (see below) that contains exactly one field: private int value; I've also implemented implicit conversion operators: public static implicit operator int(Outlet val) { return val.value; } public static implicit operator Outlet(int val) { return new Outlet(val); } I've implemented all of the following : IComparable, IComparable<Cart>, IComparable<int>, IConvertible, IEquatable<Cart>, IEquatable<int>, IFormattable I'm at a point where I really have no clue why, but whenever I serialize this object, I get no value. For instance, with XmlSerialization: <Outlet /> Also, I'm not solely concerned about XmlSerialization, I'm concerned about ALL serialization (binary for instance) How can I ensure that this serializes properly? NOTE: I did this because mapping an int,int dictionary seemed rather poorly typed to me when explicit objects with validation behavior were desired.

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• Hierarchical Data in MySQL is fast to retrieve?

  - by ajsie

  i've got a list of all countries - states - cities (- subcities/villages etc) in a XML file and to retrieve for example a state's all cities it's really quick with XML (using xml parser). i wonder, if i put all this information in mysql, is retrieving a state's all cities as fast as with XML? cause XML is designed to store hierarchical data while relational databases like mysql are not. the list contains like 500 000 entities. so i wonder if its as fast as XML using either of: Adjacency list model Nested Set model And which one should i use? Cause (theoretically) there could be unlimited levels under a state. And which is fastest for this huge dataset? Thanks!

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• Hierarchical Data in MySQL is as fast as XML to retrieve?

  - by ajsie

  i've got a list of all countries - states - cities (- subcities/villages etc) in a XML file and to retrieve for example a state's all cities it's really quick with XML (using xml parser). i wonder, if i put all this information in mysql, is retrieving a state's all cities as fast as with XML? cause XML is designed to store hierarchical data while relational databases like mysql are not. the list contains like 500 000 entities. so i wonder if its as fast as XML using either of: Adjacency list model Nested Set model And which one should i use? Cause (theoretically) there could be unlimited levels under a state (i heard that adjacency isn't good for unlimited child-levels). And which is fastest for this huge dataset? Thanks!

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• What are good hosting companies for PHP 5.3 Mysql / CouchDb / MongoDB Dev ( Lithium / CakePHP Framew

  - by Abba Bryant

  I am looking for a quality reliable host for some lithium development. I don't mind a shared platform as long as I have some ssh access. I require php 5.3.x, Mysql 5.x, and the usual imageMagick etc. Non-relational DB support up front would be nice but if they let me set one up myself I would be okay with doing it. I don't need a lot in the way of control panel tools. Good ones are appreciated but bad ones I would prefer not to even deal with. I don't anticipate needing much in the way of email but mail support would be nice to have. Cost isn't a big issue. I don't want to pay an arm and a leg but don't mind paying for what I need. Good support and decent uptime would be nice but I don't need an SLO or anything.

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• Hidden Features and Dark Corners of STL?

  - by Andrei

  C++ developers, all know the basics of C++: Declarations, conditionals, loops, operators, etc. Some of us even mastered the stuff like templates, object model, complex I/O, etc. But what are the most hidden features or tricks or dark corners of C++/STL that even C++ fans, addicts, and experts barely know? I am talking about a seasoned C++ programmer (be she/he a developer, student, fan, all three, etc), who thinks (s)he knows something 99% of us never heard or dreamed about. Something that not only makes his/her work easier, but also cool and hackish. After all, C++ is one of the most used programming languages in the world, thus it should have intricacies that only a few privileged know about and want to share with us. Boost is welcome too! One per post with an example please P.S Examples are important for other developers to copy and paste!

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• Dealing w/ Sqlite Join results in a cursor

  - by Bill

  I have a one-many relationship in my local Sqlite db. Pretty basic stuff. When I do my left outer join I get back results that look like this: the resulting cursor has multiple rows that look like this: A1.id | A1.column1 | A1.column2 | B1.a_id_fk | B1.column1 | B1.column2 A1.id | A1.column1 | A1.column2 | B2.a_id_fk | B2.column1 | B2.column2 and so on... Is there a standard practice or method of dealing with results like this ? Clearly there is only A1, but it has many B-n relationships. I am coming close to using multiple queries instead of the "relational db way". Hopefully I am just not aware of the better way to do things. I intend to expose this query via a content provider and I would hate for all of the consumers to have to write the same aggregation logic.

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• LINQ query needs either ascending or descending in the same query

  - by Sir Psycho

  Is there anyway this code can be refactored? The only difference is the order by part. Idealy I'd like to use a delegate/lamda expression so the code is reusable but I don't know how to conditionally add and remove the query operators OrderBy and OrderByDescending var linq = new NorthwindDataContext(); var query1 = linq.Customers .Where(c => c.ContactName.StartsWith("a")) .SelectMany(cus=>cus.Orders) .OrderBy(ord => ord.OrderDate) .Select(ord => ord.CustomerID); var query2 = linq.Customers .Where(c => c.ContactName.StartsWith("a")) .SelectMany(cus => cus.Orders) .OrderByDescending(ord => ord.OrderDate) .Select(ord => ord.CustomerID);

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• MySQL dual license behavior

  - by jromero

  Hi SO, I'm running a commercial(closed source) Web App development for the first time. Initially I considered MySQL the most feasible option for a DB, until I get quite confused about its dual license behavior. If I want a commercial application do I still can use the GPL version of MySQL or I must get a license? The same question in a different way: If I use MySQL's GPL version does that force me to license the whole app under GPL? Either case I would go with PostgreSQL, I just want to make really really sure about this. Even in SO I've seen related("duplicates") questions but never a clear answer... All other tools I'm gonna use to code the project are licensed under BSD or MIT. Just in case, the role of MySQL in the project is merely as relational DB to store persistent data and query it. I'd really appreciate if someone can clarify this for me. Regards, thanks in advanced.

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• C# String Operator Overloading

  - by ScottSEA

  G'Day Mates - What is the right way (excluding the argument of whether it is advisable) to overload the string operators <, , <= and = ? I've tried it five ways to Sunday and I get various errors - my best shot was declaring a partial class and overloading from there, but it won't work for some reason. namespace System { public partial class String { public static Boolean operator <(String a, String b) { return a.CompareTo(b) < 0; } public static Boolean operator >(String a, String b) { return a.CompareTo(b) > 0; } } }

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• On ocamlyacc, function application grammar and precedence

  - by Amadan

  I'm OCaml newbie and I'm trying to write a simple OCaml-like grammar, and I can't figure this out. My grammar allows something like this: let sub = fun x -> fun y -> x - y;; However, if I want to use the function so defined, I can write: (sub 7) 3 but I can't write sub 7 3, which really bugs me. For some reason, it gets interpreted as if I wrote sub (7 3) (which would treat 7 as a function with argument 3). The relevant sections are: /* other operators, then at the very end: */ %left APPLY /* ... */ expr: /* ... */ | expr expr %prec APPLY { Apply($1, $2) } Thanks!

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• Why would the assignment operator ever do something different than its matching constructor?

  - by Neil G

  I was reading some boost code, and came across this: inline sparse_vector &assign_temporary(sparse_vector &v) { swap(v); return *this; } template<class AE> inline sparse_vector &operator=(const sparse_vector<AE> &ae) { self_type temporary(ae); return assign_temporary(temporary); } It seems to be mapping all of the constructors to assignment operators. Great. But why did C++ ever opt to make them do different things? All I can think of is scoped_ptr?

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• Should I use Perl or PHP or something else for this project?

  - by Clinton

  I'm about to embark on a project that will need to: Process XML Heavy text parsing of non-xml documents Insertion of data from xml and non-xml documents into a relational DB. Present processed data to user from db using webpages. The website will be subject to short periods of very heavy loads to pages (300+ visitors a minute for several minutes), but most of the time will be idle (a dozen or so visitors a minute). The ability to cache or scale to load will be very nice. I have a very strong background in Java and web services, but I do not want to use Java for this project as I'd like to diversify my skill set. Which language would you recommend and what are some pros and cons that you might recognize from your own experiences?

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• What is the most efficient way to store a mapping "key -> event stream"?

  - by jkff

  Suppose there are ~10,000's of keys, where each key corresponds to a stream of events. I'd like to support the following operations: push(key, timestamp, event) - pushes event to the event queue for key, marked with the given timestamp. It is guaranteed that event timestamps for a particular key are pushed in sorted or almost sorted order. tail(key, timestamp) - get all events for key since the given timestamp. Usually the timestamp requests for a given key are almost monotonically increasing, almost synchronously with pushes for the same key. This stuff has to be persistent (although it is not absolutely necessary to persist pushes immediately and to keep tails with pushes strictly in sync), so I'm going to use some kind of database. What is the optimal kind of database structure for this task? Would it be better to use a relational database, a key-value storage, or something else?

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• Strategy in storing ad-hoc numbers/constants?

  - by Jiho Han

  I have a need to store a number of ad-hoc figures and constants for calculation. These numbers change periodically but they are different type of values. One might be a balance, a money amount, another might be an interest rate, and yet another might be a ratio of some kind. These numbers are then used in a calculation that involve other more structured figures. I'm not certain what the best way to store these in a relational DB is - that's the choice of storage for the app. One way, I've done before, is to create a very generic table that stores the values as text. I might store the data type along with it but the consumer knows what type it is so, in situations I didn't even need to store the data type. This kind of works fine but I am not very fond of the solution. Should I break down each of the numbers into specific categories and create tables that way? For example, create Rates table, and Balances table, etc.?

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