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• Reversible numerical calculations in Prolog

  - by user8472

  While reading SICP I came across logic programming chapter 4.4. Then I started looking into the Prolog programming language and tried to understand some simple assignments in Prolog. I found that Prolog seems to have troubles with numerical calculations. Here is the computation of a factorial in standard Prolog: f(0, 1). f(A, B) :- A > 0, C is A-1, f(C, D), B is A*D. The issues I find is that I need to introduce two auxiliary variables (C and D), a new syntax (is) and that the problem is non-reversible (i.e., f(5,X) works as expected, but f(X,120) does not). Naively, I expect that at the very least C is A-1, f(C, D) above may be replaced by f(A-1,D), but even that does not work. My question is: Why do I need to do this extra "stuff" in numerical calculations but not in other queries? I do understand (and SICP is quite clear about it) that in general information on "what to do" is insufficient to answer the question of "how to do it". So the declarative knowledge in (at least some) math problems is insufficient to actually solve these problems. But that begs the next question: How does this extra "stuff" in Prolog help me to restrict the formulation to just those problems where "what to do" is sufficient to answer "how to do it"?

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• Why does SQLite take such a long time to fetch the data?

  - by Derk

  I have two possible queries, both giving the result set I want. Query one takes about 30ms, but 150ms to fetch the data from the database. SELECT id FROM featurevalues as featval3 WHERE featval3.feature IN (?,?,?,?) AND EXISTS ( SELECT 1 FROM product_to_value, product_to_value as prod2, features, featurevalues WHERE product_to_value.feature = features.int AND product_to_value.value = featurevalues.id AND features.id = ? AND featurevalues.id IN (?,?) AND product_to_value.product = prod2.product AND prod2.value = featval3.id ) Query two takes about 3ms -this is the one I therefore prefer-, but also takes 170ms to fetch the data. SELECT ( SELECT prod2.value FROM product_to_value, product_to_value as prod2, features, featurevalues WHERE product_to_value.feature = features.int AND product_to_value.value = featurevalues.id AND features.id = ? AND featurevalues.id IN (?,?) AND product_to_value.product = prod2.product AND prod2.value = featval3.id ) as id FROM featurevalues as featval3 WHERE featval3.feature IN (?,?,?,?) The 170ms seems to be related to the number of rows from table featval3. After an index is used on featval3.feature IN (?,?,?,?), 151 items "remain" in featval3. Is there something obvious I am missing regarding the slow fetching? As far as I know everything is properly indexed.. I am confused because the second query only takes a blazing 3ms to run.

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• Extract data from PostgreSQL DB without using pg_dump

  - by John Horton

  There is a PostgreSQL database on which I only have limited access (e.g, I can't use pg_dump). I am trying to create a local "mirror" by exporting certain tables from the database. I do not have the permissions needed to just dump a table as SQL from within psql. Right now, I just have a Python script that iterates through my table_names, selects all fields and then exports them as a CSV: for table_name, file_name in zip(table_names, file_names): cmd = """echo "\\\copy (select * from %s)" to stdout WITH CSV HEADER | psql -d remote_db | gzip > ./%s/%s.gz"""%(table_name,dir_name,file_name) os.system(cmd) I would like to not use CSV if possible, as I lose the field types and the encoding can get messed up. First best would probably be some way of getting the generating SQL code for the table using \copy. Next best would be XML, ideally with some way of preserving the field types. If that doesn't work, I think the final option might be two queries---one to get the field data types, the other to get the actual data. Any thoughts or advice would be greatly appreciated - thanks!

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• How should rules for Aggregate Roots be enforced?

  - by MylesRip

  While searching the web, I came across a list of rules from Eric Evans' book that should be enforced for aggregates: The root Entity has global identity and is ultimately responsible for checking invariants Root Entities have global identity. Entities inside the boundary have local identity, unique only within the Aggregate. Nothing outside the Aggregate boundary can hold a reference to anything inside, except to the root Entity. The root Entity can hand references to the internal Entities to other objects, but they can only use them transiently (within a single method or block). Only Aggregate Roots can be obtained directly with database queries. Everything else must be done through traversal. Objects within the Aggregate can hold references to other Aggregate roots. A delete operation must remove everything within the Aggregate boundary all at once When a change to any object within the Aggregate boundary is committed, all invariants of the whole Aggregate must be satisfied. This all seems fine in theory, but I don't see how these rules would be enforced in the real world. Take rule 3 for example. Once the root entity has given an exteral object a reference to an internal entity, what's to keep that external object from holding on to the reference beyond the single method or block? (If the enforcement of this is platform-specific, I would be interested in knowing how this would be enforced within a C#/.NET/NHibernate environment.)

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• SQL Server problems reading columns with a foreign key

  - by illdev

  I have a weird situation, where simple queries seem to never finish for instance SELECT top 100 ArticleID FROM Article WHERE ProductGroupID=379114 returns immediately SELECT top 1000 ArticleID FROM Article WHERE ProductGroupID=379114 never returns SELECT ArticleID FROM Article WHERE ProductGroupID=379114 never returns SELECT top 1000 ArticleID FROM Article returns immediately By 'returning' I mean 'in query analyzer the green check mark appears and it says "Query executed successfully"'. I sometimes get the rows painted to the grid in qa, but still the query goes on waiting for my client to time out - 'sometimes': SELECT ProductGroupID AS Product23_1_, ArticleID AS ArticleID1_, ArticleID AS ArticleID18_0_, Inventory_Name AS Inventory3_18_0_, Inventory_UnitOfMeasure AS Inventory4_18_0_, BusinessKey AS Business5_18_0_, Name AS Name18_0_, ServesPeople AS ServesPe7_18_0_, InStock AS InStock18_0_, Description AS Descript9_18_0_, Description2 AS Descrip10_18_0_, TechnicalData AS Technic11_18_0_, IsDiscontinued AS IsDisco12_18_0_, Release AS Release18_0_, Classifications AS Classif14_18_0_, DistributorName AS Distrib15_18_0_, DistributorProductCode AS Distrib16_18_0_, Options AS Options18_0_, IsPromoted AS IsPromoted18_0_, IsBulkyFreight AS IsBulky19_18_0_, IsBackOrderOnly AS IsBackO20_18_0_, Price AS Price18_0_, Weight AS Weight18_0_, ProductGroupID AS Product23_18_0_, ConversationID AS Convers24_18_0_, DistributorID AS Distrib25_18_0_, type AS Type18_0_ FROM Article AS articles0_ WHERE (IsDiscontinued = '0') AND (ProductGroupID = 379121) shows this behavior. I have no idea what is going on. Probably select is broken ;) I got a foreign key on ProductGroups ALTER TABLE [dbo].[Article] WITH CHECK ADD CONSTRAINT [FK_ProductGroup_Articles] FOREIGN KEY([ProductGroupID]) REFERENCES [dbo].[ProductGroup] ([ProductGroupID]) GO ALTER TABLE [dbo].[Article] CHECK CONSTRAINT [FK_ProductGroup_Articles] there are some 6000 rows and IsDiscontinued is a bit, not null, but leaving this condition out does not change the outcome. Anyone can tell me how to handle such a situation? More info, anyone? Additional Info: this does not seem to be restricted to this Foreign Key, but all/some referencing this entity.

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• Codeigniter Inserting Multidimensional Array as rows in MySQL

  - by RisingSun

  Please Refer to this question I asked Codeigniter Insert Multiple Rows in SQL To restate <tr> <td><input type="text" name="user[0][name]" value=""></td> <td><input type="text" name="user[0][address]" value=""><br></td> <td><input type="text" name="user[0][age]" value=""></td> <td><input type="text" name="user[0][email]" value=""></td> </tr> <tr> <td><input type="text" name="user[1][name]" value=""></td> <td><input type="text" name="user[1][address]" value=""><br></td> <td><input type="text" name="user[1][age]" value=""></td> <td><input type="text" name="user[1][email]" value=""></td> </tr> .......... Can Be Inserted into MySQL as this foreach($_POST['user'] as $user) { $this->db->insert('mytable', $user); } This results in multiple MySQL queries. Is it possible to optimise it further, so that the insert occurs in one query Something like this insert multiple rows via a php array into mysql but taking advantage of codeigniters simpler syntax. Thanks

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• Creating a RESTful API - HELP!

  - by Martin Cox

  Hi Chaps Over the last few weeks I've been learning about iOS development, which has naturally led me into the world of APIs. Now, searching around on the Internet, I've come to the conclusion that using the REST architecture is very much recommended - due to it's supposed simplicity and ease of implementation. However, I'm really struggling with the implementation side of REST. I understand the concept; using HTTP methods as verbs to describe the action of a request and responding with suitable response codes, and so on. It's just, I don't understand how to code it. I don't get how I map a URI to an object. I understand that a GET request for domain.com/api/user/address?user_id=999 would return the address of user 999 - but I don't understand where or how that mapping from /user/address to some method that queries a database has taken place. Is this all coded in one php script? Would I just have a method that grabs the URI like so: $array = explode("/", ltrim(rtrim($_SERVER['REQUEST_URI'], "/"), "/")) And then cycle through that array, first I would have a request for a "user", so the PHP script would direct my request to the user object and then invoke the address method. Is that what actually happens? I've probably not explained my thought process very well there. The main thing I'm not getting is how that URI /user/address?id=999 somehow is broken down and executed - does it actually resolve to code? class user(id) { address() { //get user address } } I doubt I'm making sense now, so I'll call it a day trying to explain further. I hope someone out there can understand what I'm trying to say! Thanks Chaps, look forward to your responses. Martin p.s - I'm not a developer yet, I'm learning :)

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• Access 2007 not allowing user to delete record in subform

  - by Todd McDermid

  Good day... The root of my issue is that there's no context menu allowing the user to delete a row from a form. The "delete" button on the ribbon is also disabled. In Access 2003, apparently this function was available, but since our recent "upgrade" to 2007 (file is still in MDB format) it's no longer there. Please keep in mind I'm not an Access dev, nor did I create this app - I inherited support for it. ;) Now for the details, and what I've tried. The form in question is a subform on a larger form. I've tried turning "AllowDeletes" on on both forms. I've checked the toolbar and ribbon properties on the forms to see if they loaded some custom stuff, but no. I've tried changing the "record locks" to "on edit", no joy. I examined the query to see if it was "too complicated" to permit a delete - as far as I can tell, it's a very simple two (linked) table join. Compared to another form in this app that does permit row deletes, it has a much more complicated (multi-join, built on queries) query. Is there a resource that would describe the required conditions for allowing deletes? Thanks in advance...

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• How to filter Many2Many / Generic Relations properly with Q?

  - by HWM-Rocker

  Hi, I have 3 Models, the TaggedObject has a GenericRelation with the ObjectTagBridge. And the ObjectTagBridge has a ForeignKey to the Tag Model. class TaggedObject(models.Model): """ class that represent a tagged object """ tags = generic.GenericRelation('ObjectTagBridge', blank=True, null=True) class ObjectTagBridge(models.Model): """ Help to connect a generic object to a Tag. """ # pylint: disable-msg=W0232,R0903 content_type = models.ForeignKey(ContentType) object_id = models.PositiveIntegerField() content_object = generic.GenericForeignKey('content_type', 'object_id') tag = models.ForeignKey('Tag') class Tag(models.Model): ... when I am attaching a Tag to an Object, I am creating a new ObjectTagBridge and set its ForeignKey tag to the Tag I want to attach. That is working fine, and I can get all Tags that I attached to my Object very easy. But when I want to get (filter) all Objects that have Tag1 and Tag2 I tried to something like this: query = Q(tags__tag=Tag1) & Q(tags__tag=Tag2) object_list = TaggedObjects.filter(query) but now my object_list is empty, because it is looking for TaggedObjects that have one ObjectTagBridge with 2 tag objects, the first with Tag1 and the second with Tag2. I my application will be more complex Q queries than this one, so I think I need a solution with this Q object. In fact any combination of binary conjunctions, like: (...) and ( (...) or not(...)) How can I filter this correctly? Every answer is welcome, maybe there is also a different way do achieve this. thx for your help!!!

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• Fetching just the Key/id from a ReferenceProperty in App Engine

  - by ozone

  Hi SO, I could use a little help in AppEngine land... Using the [Python] API I create relationships like this example from the docs: class Author(db.Model): name = db.StringProperty() class Story(db.Model): author = db.ReferenceProperty(Author) story = db.get(story_key) author_name = story.author.name As I understand it, that example will make two datastore queries. One to fetch the Story and then one to deference the Author inorder to access the name. But I want to be able to fetch the id, so do something like: story = db.get(story_key) author_id = story.author.key().id() I want to just get the id from the reference. I do not want to have to deference (therefore query the datastore) the ReferenceProperty value. From reading the documentation it says that the value of a ReferenceProperty is a Key Which leads me to think that I could just call .id() on the reference's value. But it also says: The ReferenceProperty model provides features for Key property values such as automatic dereferencing. I can't find anything that explains when this referencing takes place? Is it safe to call .id() on the ReferenceProperty's value? Can it be assumed that calling .id() will not cause a datastore lookup?

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• GROUP BY ID range?

  - by d0ugal

  Given a data set like this; +-----+---------------------+--------+ | id | date | result | +-----+---------------------+--------+ | 121 | 2009-07-11 13:23:24 | -1 | | 122 | 2009-07-11 13:23:24 | -1 | | 123 | 2009-07-11 13:23:24 | -1 | | 124 | 2009-07-11 13:23:24 | -1 | | 125 | 2009-07-11 13:23:24 | -1 | | 126 | 2009-07-11 13:23:24 | -1 | | 127 | 2009-07-11 13:23:24 | -1 | | 128 | 2009-07-11 13:23:24 | -1 | | 129 | 2009-07-11 13:23:24 | -1 | | 130 | 2009-07-11 13:23:24 | -1 | | 131 | 2009-07-11 13:23:24 | -1 | | 132 | 2009-07-11 13:23:24 | -1 | | 133 | 2009-07-11 13:23:24 | -1 | | 134 | 2009-07-11 13:23:24 | -1 | | 135 | 2009-07-11 13:23:24 | -1 | | 136 | 2009-07-11 13:23:24 | -1 | | 137 | 2009-07-11 13:23:24 | -1 | | 138 | 2009-07-11 13:23:24 | 1 | | 139 | 2009-07-11 13:23:24 | 0 | | 140 | 2009-07-11 13:23:24 | -1 | +-----+---------------------+--------+ How would I go about grouping the results by day 5 records at a time. The above results is part of the live data, there is over 100,000 results rows in the table and its growing. Basically I want to measure the change over time, so want to take a SUM of the result every X records. In the real data I'll be doing it ever 100 or 1000 but for the data above perhaps every 5. If i could sort it by date I would do something like this; SELECT DATE_FORMAT(date, '%h%i') ym, COUNT(result) 'Total Games', SUM(result) as 'Score' FROM nn_log GROUP BY ym; I can't figure out a way of doing something similar with numbers. The order is sorted by the date but I hope to split the data up every x results. It's safe to assume there are no blank rows. Doing it above with the data you could do multiple selects like; SELECT SUM(result) FROM table LIMIT 0,5; SELECT SUM(result) FROM table LIMIT 5,5; SELECT SUM(result) FROM table LIMIT 10,5; Thats obviously not a very good way to scale up to a bigger problem. I could just write a loop but I'd like to reduce the number of queries.

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• How to create view model without sorting collections in memory.

  - by Chevex

  I have a view model (below). public class TopicsViewModel { public Topic Topic { get; set; } public Reply LastReply { get; set; } } I want to populate an IQueryable<TopicsViewModel> with values from my IQueryable<Topic> collection and IQueryable<Reply> collection. I do not want to use the attached entity collection (i.e. Topic.Replies) because I only want the last reply for that topic and doing Topic.Replies.Last() loads the entire entity collection in memory and then grabs the last one in the list. I am trying to stay in IQueryable so that the query is executed in the database. I also don't want to foreach through topics and query replyRepository.Replies because looping through IQueryable<Topic> will start the lazy loading. I'd prefer to build one expression and have all the leg work done in the lower layers. I have the following: IQueryable<TopicsViewModel> topicsViewModel = from x in topicRepository.Topics from y in replyRepository.Replies where y.TopicID == x.TopicID orderby y.PostedDate ascending select new TopicsViewModel { Topic = x, LastReply = y }; But this isn't working. Any ideas how I can populate an IQueryable or IEnumerable of TopicsViewModel so that it queries the database and grabs topics and that topic's last reply? I am trying really hard to avoid grabbing all replies related to that topic. I only want to grab the last reply. Thank you for any insight you have to offer.

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• How to track auto-generated id's in select-insert statement

  - by k rey

  I have two tables detail and head. The detail table will be written first. Later, the head table will be written. The head is a summary of the detail table. I would like to keep a reference from the detail to the head table. I have a solution but it is not elegant and requires duplicating the joins and filters that were used during summation. I am looking for a better solution. The below is an example of what I currently have. In this example, I have simplified the table structure. In the real world, the summation is very complex. -- Preparation create table #detail ( detail_id int identity(1,1) , code char(4) , amount money , head_id int null ); create table #head ( head_id int identity(1,1) , code char(4) , subtotal money ); insert into #detail ( code, amount ) values ( 'A', 5 ); insert into #detail ( code, amount ) values ( 'A', 5 ); insert into #detail ( code, amount ) values ( 'B', 2 ); insert into #detail ( code, amount ) values ( 'B', 2 ); -- I would like to somehow simplify the following two queries insert into #head ( code, subtotal ) select code, sum(amount) from #detail group by code update #detail set head_id = h.head_id from #detail d inner join #head h on d.code = h.code -- This is the desired end result select * from #detail Desired end result of detail table: detail_id code amount head_id 1 A 5.00 1 2 A 5.00 1 3 B 2.00 2 4 B 2.00 2

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• Linq generic Expression in query on "element" or on IQueryable (multiple use)

  - by Bogdan Maxim

  Hi, I have the following expression public static Expression<Func<T, bool>> JoinByDateCheck<T>(T entity, DateTime dateToCheck) where T : IDateInterval { return (entityToJoin) => entityToJoin.FromDate.Date <= dateToCheck.Date && (entityToJoin.ToDate == null || entityToJoin.ToDate.Value.Date >= dateToCheck.Date); } IDateInterval interface is defined like this: interface IDateInterval { DateTime FromDate {get;} DateTime? ToDate {get;} } and i need to apply it in a few ways: (1) Query on Linq2Sql Table: var q1 = from e in intervalTable where FunctionThatCallsJoinByDateCheck(e, constantDateTime) select e; or something like this: intervalTable.Where(FunctionThatCallsJoinByDateCheck(e, constantDateTime)) (2) I need to use it in some table joins (as linq2sql doesn't provide comparative join): var q2 = from e1 in t1 join e2 in t2 on e1.FK == e2.PK where OtherFunctionThatCallsJoinByDateCheck(e2, e1.FromDate) or var q2 = from e1 in t1 from e2 in t2 where e1.FK == e2.PK && OtherFunctionThatCallsJoinByDateCheck(e2, e1.FromDate) (3) I need to use it in some queries like this: var q3 = from e in intervalTable.FilterFunctionThatCallsJoinByDateCheck(constantDate); Dynamic linq is not something that I can use, so I have to stick to plain linq. Thank you Clarification: Initially I had just the last method (FilterFunctionThatCallsJoinByDateCheck(this IQueryable<IDateInterval> entities, DateTime dateConstant) ) that contained the code from the expression. The problem is that I get a SQL Translate exception if I write the code in a method and call it like that. All I want is to extend the use of this function to the where clause (see the second query in point 2)

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• What are the advantages of a query using a derived table(s) over a query not using them?

  - by AspOnMyNet

  I know how derived tables are used, but I still can’t really see any real advantages of using them. For example, in the following article http://techahead.wordpress.com/2007/10/01/sql-derived-tables/ the author tried to show benefits of a query using derived table over a query without one with an example, where we want to generate a report that shows off the total number of orders each customer placed in 1996, and we want this result set to include all customers, including those that didn’t place any orders that year and those that have never placed any orders at all( he’s using Northwind database ). But when I compare the two queries, I fail to see any advantages of a query using a derived table ( if nothing else, use of a derived table doesn't appear to simplify our code, at least not in this example): Regular query: SELECT C.CustomerID, C.CompanyName, COUNT(O.OrderID) AS TotalOrders FROM Customers C LEFT OUTER JOIN Orders O ON C.CustomerID = O.CustomerID AND YEAR(O.OrderDate) = 1996 GROUP BY C.CustomerID, C.CompanyName Query using a derived table: SELECT C.CustomerID, C.CompanyName, COUNT(dOrders.OrderID) AS TotalOrders FROM Customers C LEFT OUTER JOIN (SELECT * FROM Orders WHERE YEAR(Orders.OrderDate) = 1996) AS dOrders ON C.CustomerID = dOrders.CustomerID GROUP BY C.CustomerID, C.CompanyName Perhaps this just wasn’t a good example, so could you show me an example where benefits of derived table are more obvious? thanx

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• Trouble creating a SQL query

  - by JoBu1324

  I've been thinking about how to compose this SQL query for a while now, but after thinking about it for a few hours I thought I'd ask the SO community to see if they have any ideas. Here is a mock up of the relevant portion of the tables: contracts id date ar (yes/no) term payments contract_id payment_date The object of the query is to determine, per month, how many payments we expect, vs how many payments we received. conditions for expecting a payment Expected payments begin on contracts.term months after contracts.date, if contracts.ar is "yes". Payments continue to be expected until the month after the first missed payment. There is one other complication to this: payments might be late, but they need to show up as if they were paid on the date expected. The data is all there, but I've been having trouble wrapping my head around the SQL query. I am not an SQL guru - I merely have a decent amount of experience handling simpler queries. I'd like to avoid filtering the results in code, if possible - but without your help that may be what I have to do. Expected Output Month Expected Payments Received Payments January 500 450 February 498 478 March 234 211 April 987 789 ... SQL Fiddle I've created an SQL Fiddle: http://sqlfiddle.com/#!2/a2c3f/2

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• Passing a WHERE clause for a Linq-to-Sql query as a parameter

  - by Mantorok

  Hi all This is probably pushing the boundaries of Linq-to-Sql a bit but given how versatile it has been so far I thought I'd ask. I have 3 queries that are selecting identical information and only differ in the where clause, now I know I can pass a delegate in but this only allows me to filter the results already returned, but I want to build up the query via parameter to ensure efficiency. Here is the query: from row in DataContext.PublishedEvents join link in DataContext.PublishedEvent_EventDateTimes on row.guid equals link.container join time in DataContext.EventDateTimes on link.item equals time.guid where row.ApprovalStatus == "Approved" && row.EventType == "Event" && time.StartDate <= DateTime.Now.Date.AddDays(1) && (!time.EndDate.HasValue || time.EndDate.Value >= DateTime.Now.Date.AddDays(1)) orderby time.StartDate select new EventDetails { Title = row.EventName, Description = TrimDescription(row.Description) }; The code I want to apply via a parameter would be: time.StartDate <= DateTime.Now.Date.AddDays(1) && (!time.EndDate.HasValue || time.EndDate.Value >= DateTime.Now.Date.AddDays(1)) Is this possible? I don't think it is but thought I'd check out first. Thanks

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• Dealing with SQLException with spring,hibernate & Postgres

  - by mad

  Hi im working on a project using HibernateDaoSUpport from my Daos from Spring & spring-ws & hibernate & postgres who will be used in a national application (means a lot of users) Actually, every exception from hibernate is automatically transformed into some specific Spring dataAccesException. I have a table with a keyword on the dabatase & a unique constraint on the keywords : no duplicate keywords is allowed. I have found twows ways to deal with with that in the Insert Dao: 1- Check for the duplicate manually (with a select) prior to doing your insert. I means that the spring transaction will have a SERIALIZABLE isolation level. The obvious drawback is that we have now 2 queries for a simple insert.Advantage: independent of the database 2-let the insert gone & catch the SqlException & convert it to a userfriendly message & errorcode to the final consumer of our webservices. Solution 2: Spring has developped a way to translate specific exeptions into customized exceptions. see http://www.oracle.com/technology/pub/articles/marx_spring.html In my case i would have a ConstraintViolationException. Ideally i would like to write a custom SQLExceptionTranslator to map the duplicate word constraint in the database with a DuplicateWordException. But i can have many unique constraints on the same table. So i have to get the message of the SQLEXceptions in order to find the name of the constraint declared in the create table "uq_duplicate-constraint" for example. Now i have a strong dependency with the database. Thanks in advance for your answers & excuse me for my poor english (it is not my mother tongue)

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• Numerical calculations in Prolog

  - by user8472

  While reading SICP I came across logic programming chapter 4.4. Then I started looking into the Prolog programming language and tried to understand some simple assignments in Prolog. I found that Prolog seems to have troubles with numerical calculations. Here is the computation of a factorial in standard Prolog: f(0, 1). f(A, B) :- A > 0, C is A-1, f(C, D), B is A*D. The issues I find is that I need to introduce two auxiliary variables (C and D), a new syntax (is) and that the problem is non-reversible (i.e., f(5,X) works as expected, but f(X,120) does not). Naively, I expect that at the very least C is A-1, f(C, D) above may be replaced by f(A-1,D), but even that does not work. My question is: Why do I need to do this extra "stuff" in numerical calculations but not in other queries? I do understand (and SICP is quite clear about it) that in general information on "what to do" is insufficient to answer the question of "how to do it". So the declarative knowledge in (at least some) math problems is insufficient to actually solve these problems. But that begs the next question: How does this extra "stuff" in Prolog help me to restrict the formulation to just those problems where "what to do" is sufficient to answer "how to do it"?

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• Linq2Sql: query - subquery optimisation

  - by Budda

  I have the following query: IList<InfrStadium> stadiums = (from sector in DbContext.sectors where sector.Type=typeValue select new InfrStadium(sector.TeamId) ).ToList(); and InfrStadium class constructor: private InfrStadium(int teamId) { IList<Sector> teamSectors = (from sector in DbContext.sectors where sector.TeamId==teamId select sector) .ToList<>(); ... work with data } Current implementation perform 1+n queries, where n - number of records fetched the 1st time. I want to optimize that. And another one I would love to do using 'group' operator in way like this: IList<InfrStadium> stadiums = (from sector in DbContext.sectors group sector by sector.TeamId into team_sectors select new InfrStadium(team_sectors.Key, team_sectors) ).ToList(); with appropriate constructor: private InfrStadium(int iTeamId, IEnumerable<InfrStadiumSector> eSectors) { IList<Sector> teamSectors = eSectors.ToList(); ... work with data } But attempt to launch query causes the following error: Expression of type 'System.Int32' cannot be used for constructor parameter of type 'System.Collections.Generic.IEnumerable`1[InfrStadiumSector]' Question 1: Could you please explain, what is wrong here, I don't understand why 'team_sectors' is applied as 'System.Int32'? I've tried to change query a little (replace IEnumerable with IQueryeable): IList<InfrStadium> stadiums = (from sector in DbContext.sectors group sector by sector.TeamId into team_sectors select new InfrStadium(team_sectors.Key, team_sectors.AsQueryable()) ).ToList(); with appropriate constructor: private InfrStadium(int iTeamId, IQueryeable<InfrStadiumSector> eSectors) { IList<Sector> teamSectors = eSectors.ToList(); ... work with data } In this case I've received another but similar error: Expression of type 'System.Int32' cannot be used for parameter of type 'System.Collections.Generic.IEnumerable1[InfrStadiumSector]' of method 'System.Linq.IQueryable1[InfrStadiumSector] AsQueryableInfrStadiumSector' Question 2: Actually, the same question: can't understand at all what is going on here... P.S. I have another to optimize query idea (describe here: Linq2Sql: query optimisation) but I would love to find a solution with 1 request to DB).

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• EF/LINQ: Where() against a property of a subtype

  - by ladenedge

  I have a set of POCOs, all of which implement the following simple interface: interface IIdObject { int Id { get; set; } } A subset of these POCOs implement this additional interface: interface IDeletableObject : IIdObject { bool IsDeleted { get; set; } } I have a repository hierarchy that looks something like this: IRepository<T <: BasicRepository<T <: ValidatingRepository<T (where T is IIdObject) I'm trying to add a FilteringRepository to the hierarchy such that all of the POCOs that implement IDeletableObject have a Where(p => p.IsDeleted == false) filter applied before any other queries take place. My goal is to avoid duplicating the hierarchy solely for IDeletableObjects. My first attempt looked like this: public override IQueryable<T> Query() { return base.Query().Where(t => ((IDeletableObject)t).IsDeleted == false); } This works well with LINQ to Objects, but when I switch to an EF backend I get: "LINQ to Entities only supports casting Entity Data Model primitive types." I went on to try some fancier parameterized solutions, but they ultimately failed because I couldn't make T covariant in the following case for some reason I don't quite understand: interface IQueryFilter<out T> // error { Expression<Func<T, bool>> GetFilter(); } I'd be happy to go into more detail on my more complicated solutions if it would help, but I think I'll stop here for now in hope that someone might have an idea for me to try. Thanks very much in advance!

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• query structure - ignoring entries for the same event from multiple users?

  - by Andrew Heath

  One table in my MySQL database tracks game plays. It has the following structure: SCENARIO_VICTORIES [ID] [scenario_id] [game] [timestamp] [user_id] [winning_side] [play_date] ID is the autoincremented primary key. timestamp records the moment of submission for the record. winning_side has one of three possible values: 1, 2, or 0 (meaning a draw) One of the queries done on this table calculates the victory percentage for each scenario, when that scenario's page is viewed. The output is expressed as: Side 1 win % Side 2 win % Draw % and queried with: SELECT winning_side, COUNT(scenario_id) FROM scenario_victories WHERE scenario_id='$scenID' GROUP BY winning_side ORDER BY winning_side ASC and then processed into the percentages and such. Sorry for the long setup. My problem is this: several of my users play each other, and record their mutual results. So these battles are being doubly represented in the victory percentages and result counts. Though this happens infrequently, the userbase isn't large and the double entries do have a noticeable effect on the data. Given the table and query above - does anyone have any suggestions for how I can "collapse" records that have the same play_date & game & scenario_id & winning_side so that they're only counted once?

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• Do database engines other than SQL Server behave this way?

  - by Yishai

  I have a stored procedure that goes something like this (pseudo code) storedprocedure param1, param2, param3, param4 begin if (param4 = 'Y') begin select * from SOME_VIEW order by somecolumn end else if (param1 is null) begin select * from SOME_VIEW where (param2 is null or param2 = SOME_VIEW.Somecolumn2) and (param3 is null or param3 = SOME_VIEW.SomeColumn3) order by somecolumn end else select somethingcompletelydifferent end All ran well for a long time. Suddenly, the query started running forever if param4 was 'Y'. Changing the code to this: storedprocedure param1, param2, param3, param4 begin if (param4 = 'Y') begin set param2 = null set param3 = null end if (param1 is null) begin select * from SOME_VIEW where (param2 is null or param2 = SOME_VIEW.Somecolumn2) and (param3 is null or param3 = SOME_VIEW.SomeColumn3) order by somecolumn end else select somethingcompletelydifferent And it runs again within expected parameters (15 seconds or so for 40,000+ records). This is with SQL Server 2005. The gist of my question is this particular "feature" specific to SQL Server, or is this a common feature among RDBMS' in general that: Queries that ran fine for two years just stop working as the data grows. The "new" execution plan destroys the ability of the database server to execute the query even though a logically equivalent alternative runs just fine? This may seem like a rant against SQL Server, and I suppose to some degree it is, but I really do want to know if others experience this kind of reality with Oracle, DB2 or any other RDBMS. Although I have some experience with others, I have only seen this kind of volume and complexity on SQL Server, so I'm curious if others with large complex databases have similar experience in other products.

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• django dynamically deduce SITE_ID according to the domain

  - by dcrodjer

  I am trying to develop a site which will render multiple customized sites according to the domain name (subdomain to be more precise). My all the domain names are redirected to the So for each site there will be a corresponding model which defines how the site should look (SITE - SITE_SETTINGS) What will be the best way to utilize the django sites framework to get the SITE_ID of the current site from the domain name instead of hard-coding it in the settings files (django sites documentation) and run database queries, render the views accordingly? If using multiple settings file is my only option can this (wsgi script handle domain name) be done? Update So finally, following lukes answer, what I will do is define a custom middleware which makes the views available with the important vars required according to the domain. And as far as sitemaps and comments is concerned, I will have to customize sitemaps app and a custom sites model on which the other models of sites will be based. And since the comments system is based on the hard-coded sitemap ID I can use it just as is on the models (models will already be filtered according to the site based on my sites framework) though the permalink feature will have to be customized. So a lot of customization. Please suggest if I am going wrong anywhere in this because I have to ensure that the features of the project are optimized. Thanks!

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• MySQL move data from one table to another, matching ID's

  - by Reveller

  I have (a.o.) two MySQL tables with (a.o.) the following columns: tweets: ------------------------------------- id text from_user_id from_user ------------------------------------- 1 Cool tweet! 13295354 tradeny 2 Tweeeeeeeet 43232544 bolleke 3 Yet another 13295354 tradeny 4 Something.. 53546443 janusz4 users: ------------------------------------- id from_user num_tweets from_user_id ------------------------------------- 1 tradeny 2235 2 bolleke 432 3 janusz4 5354 I now want to normalize the tweets table, replacing tweets.from_user with an integer that matches users.id. Secondly, I want to fill in the corresponding users.from_user_id, Finally, I want to delete tweets.from_user_id so that the end result would look like: tweets: ------------------------ id text from_user ------------------------ 1 Cool tweet! 1 2 Tweeeeeeeet 2 3 Yet another 1 4 Something.. 3 users: ------------------------------------- id from_user num_tweets from_user_id ------------------------------------- 1 tradeny 2235 13295354 2 bolleke 432 43232544 3 janusz4 5354 53546443 My question is whether one could help me form the proper queries for this. I have only come so far: UPDATE tweets SET from_user = (SELECT id FROM users WHERE from_user = tweets.from_user) WHERE... UPDATE users SET from_user_id = (SELECT from_user_id FROM tweets WHERE from_user = tweets.from_user) WHERE... ALTER TABLE tweets DROP from_user_id Any help would be greatly appreciated :-)

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