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• how to avoid sub-query to gain performance

  - by chun

  hi i have a reporting query which have 2 long sub-query SELECT r1.code_centre, r1.libelle_centre, r1.id_equipe, r1.equipe, r1.id_file_attente, r1.libelle_file_attente,r1.id_date, r1.tranche, r1.id_granularite_de_periode,r1.granularite, r1.ContactsTraites, r1.ContactsenParcage, r1.ContactsenComm, r1.DureeTraitementContacts, r1.DureeComm, r1.DureeParcage, r2.AgentsConnectes, r2.DureeConnexion, r2.DureeTraitementAgents, r2.DureePostTraitement FROM ( SELECT cc.id_centre_contact, cc.code_centre, cc.libelle_centre, a.id_equipe, a.equipe, a.id_file_attente, f.libelle_file_attente, a.id_date, g.tranche, g.id_granularite_de_periode, g.granularite, sum(Nb_Contacts_Traites) as ContactsTraites, sum(Nb_Contacts_en_Parcage) as ContactsenParcage, sum(Nb_Contacts_en_Communication) as ContactsenComm, sum(Duree_Traitement/1000) as DureeTraitementContacts, sum(Duree_Communication / 1000 + Duree_Conference / 1000 + Duree_Com_Interagent / 1000) as DureeComm, sum(Duree_Parcage/1000) as DureeParcage FROM agr_synthese_activite_media_fa_agent a, centre_contact cc, direction_contact dc, granularite_de_periode g, media m, file_attente f WHERE m.id_media = a.id_media AND cc.id_centre_contact = a.id_centre_contact AND a.id_direction_contact = dc.id_direction_contact AND dc.direction_contact ='INCOMING' AND a.id_file_attente = f.id_file_attente AND m.media = 'PHONE' AND ( ( g.valeur_min = date_format(a.id_date,'%d/%m') and g.granularite = 'Jour') or ( g.granularite = 'Heure' and a.id_th_heure = g.id_granularite_de_periode) ) GROUP by cc.id_centre_contact, a.id_equipe, a.id_file_attente, a.id_date, g.tranche, g.id_granularite_de_periode) r1, ( (SELECT cc.id_centre_contact,cc.code_centre, cc.libelle_centre, a.id_equipe, a.equipe, a.id_date, g.tranche, g.id_granularite_de_periode,g.granularite, count(distinct a.id_agent) as AgentsConnectes, sum(Duree_Connexion / 1000) as DureeConnexion, sum(Duree_en_Traitement / 1000) as DureeTraitementAgents, sum(Duree_en_PostTraitement / 1000) as DureePostTraitement FROM activite_agent a, centre_contact cc, granularite_de_periode g WHERE ( g.valeur_min = date_format(a.id_date,'%d/%m') and g.granularite = 'Jour') AND cc.id_centre_contact = a.id_centre_contact GROUP BY cc.id_centre_contact, a.id_equipe, a.id_date, g.tranche, g.id_granularite_de_periode ) UNION (SELECT cc.id_centre_contact,cc.code_centre, cc.libelle_centre, a.id_equipe, a.equipe, a.id_date, g.tranche, g.id_granularite_de_periode,g.granularite, count(distinct a.id_agent) as AgentsConnectes, sum(Duree_Connexion / 1000) as DureeConnexion, sum(Duree_en_Traitement / 1000) as DureeTraitementAgents, sum(Duree_en_PostTraitement / 1000) as DureePostTraitement FROM activite_agent a, centre_contact cc, granularite_de_periode g WHERE ( g.granularite = 'Heure' AND a.id_th_heure = g.id_granularite_de_periode) AND cc.id_centre_contact = a.id_centre_contact GROUP BY cc.id_centre_contact,a.id_equipe, a.id_date, g.tranche, g.id_granularite_de_periode) ) r2 WHERE r1.id_centre_contact = r2.id_centre_contact AND r1.id_equipe = r2.id_equipe AND r1.id_date = r2.id_date AND r1.tranche = r2.tranche AND r1.id_granularite_de_periode = r2.id_granularite_de_periode GROUP BY r1.id_centre_contact , r1.id_equipe, r1.id_file_attente, r1.id_date, r1.tranche, r1.id_granularite_de_periode ORDER BY r1.code_centre, r1.libelle_centre, r1.equipe, r1.libelle_file_attente, r1.id_date, r1.id_granularite_de_periode,r1.tranche the EXPLAIN shows | id | select_type | table | type| possible_keys | key | key_len | ref| rows | Extra | '1', 'PRIMARY', '<derived3>', 'ALL', NULL, NULL, NULL, NULL, '2520', 'Using temporary; Using filesort' '1', 'PRIMARY', '<derived2>', 'ALL', NULL, NULL, NULL, NULL, '4378', 'Using where; Using join buffer' '3', 'DERIVED', 'a', 'ALL', 'fk_Activite_Agent_centre_contact', NULL, NULL, NULL, '83433', 'Using temporary; Using filesort' '3', 'DERIVED', 'g', 'ref', 'Index_granularite,Index_Valeur_min', 'Index_Valeur_min', '23', 'func', '1', 'Using where' '3', 'DERIVED', 'cc', 'ALL', 'PRIMARY', NULL, NULL, NULL, '6', 'Using where; Using join buffer' '4', 'UNION', 'g', 'ref', 'PRIMARY,Index_granularite', 'Index_granularite', '23', '', '24', 'Using where; Using temporary; Using filesort' '4', 'UNION', 'a', 'ref', 'fk_Activite_Agent_centre_contact,fk_activite_agent_TH_heure', 'fk_activite_agent_TH_heure', '5', 'reporting_acd.g.Id_Granularite_de_periode', '2979', 'Using where' '4', 'UNION', 'cc', 'ALL', 'PRIMARY', NULL, NULL, NULL, '6', 'Using where; Using join buffer' NULL, 'UNION RESULT', '<union3,4>', 'ALL', NULL, NULL, NULL, NULL, NULL, '' '2', 'DERIVED', 'g', 'range', 'PRIMARY,Index_granularite,Index_Valeur_min', 'Index_granularite', '23', NULL, '389', 'Using where; Using temporary; Using filesort' '2', 'DERIVED', 'a', 'ALL', 'fk_agr_synthese_activite_media_fa_agent_centre_contact,fk_agr_synthese_activite_media_fa_agent_direction_contact,fk_agr_synthese_activite_media_fa_agent_file_attente,fk_agr_synthese_activite_media_fa_agent_media,fk_agr_synthese_activite_media_fa_agent_th_heure', NULL, NULL, NULL, '20903', 'Using where; Using join buffer' '2', 'DERIVED', 'cc', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'reporting_acd.a.Id_Centre_Contact', '1', '' '2', 'DERIVED', 'f', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'reporting_acd.a.Id_File_Attente', '1', '' '2', 'DERIVED', 'dc', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'reporting_acd.a.Id_Direction_Contact', '1', 'Using where' '2', 'DERIVED', 'm', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'reporting_acd.a.Id_Media', '1', 'Using where' don't know it very clear, but i think is the problem of seems it take full scaning than i change all the sub-query to views(create view as select sub-query), and the result is the same thanks for any advice

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• Database indexes and their Big-O notation

  - by miket2e

  I'm trying to understand the performance of database indexes in terms of Big-O notation. Without knowing much about it, I would guess that: Querying on a primary key or unique index will give you a O(1) lookup time. Querying on a non-unique index will also give a O(1) time, albeit maybe the '1' is slower than for the unique index (?) Querying on a column without an index will give a O(N) lookup time (full table scan). Is this generally correct ? Will querying on a primary key ever give worse performance than O(1) ? My specific concern is for SQLite, but I'd be interested in knowing to what extent this varies between different databases too.

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• SQLAlchemy & Complex Queries

  - by user356594

  I have to implement ACL for an existing application. So I added the a user, group and groupmembers table to the database. I defined a ManyToMany relationship between user and group via the association table groupmembers. In order to protect some ressources of the app (i..e item) I added a additional association table auth_items which should be used as an association table for the ManyToMany relationship between groups/users and the specific item. item has following columns: user_id -- user table group_id -- group table item_id -- item table at least on of user_id and group_id columns are set. So it's possible to define access for a group or for a user to a specific item. I have used the AssociationProxy to define the relationship between users/groups and items. I now want to display all items which the user has access to and I have a really hard time doing that. Following criteria are used: All items which are owned by the user should be shown (item.owner_id = user.id) All public items should be shown (item.access = public) All items which the user has access to should be shown (auth_item.user_id = user.id) All items which the group of the user has access to should be shown. The first two criteria are quite straightforward, but I have a hard time doing the 3rd one. Here is my approach: clause = and_(item.access == 'public') if user is not None: clause = or_(clause,item.owner == user,item.users.contains(user),item.groups.contains(group for group in user.groups)) The third criteria produces an error. item.groups.contains(group for group in user.groups) I am actually not sure if this is a good approach at all. What is the best approach when filtering manytomany relationships? How I can filter a manytomany relationship based on another list/relationship? Btw I am using the latest sqlalchemy (6.0) and elixir version Thanks for any insights.

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• How Can I Reference Multiple Primary Keys For A Vector Type Primary Key

  - by AndreiC

  Hi, I have the following scenario: a table of projects and a table of persons, working on one or serveral projects. Also, I have a project id column (of type int), in the first table, which is a primary key there and I have a vector of project ids, as a column of type int, in my second table (persons), that references primary keys from the first table. What is the correct syntax for referencing multiple primary keys, from a vector foreign key. This is the way I am trying to create the table, but I am not sure what to place at the indicated line: CREATE TABLE Persons( Person_Id int, ..., ProjectsList int[], FOREIGN KEY (ProjectsList) REFERENCES Projects(Project_id) -- not sure what how to define the link here ); I hope my explanations are not over-complicated. Thank you in advance for helping!

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• How to select parent row only if has at least one child?

  - by Matt McCormick

  I have a simple one-to-many relationship. I would like to select rows from the parent only when they have at least one child. So, if there are no children, then the parent row is not returned in the result set. Eg. Parent: +--+---------+ |id| text | +--+---------+ | 1| Blah | | 2| Blah2 | | 3| Blah3 | +--+---------+ Children +--+------+-------+ |id|parent| other | +--+------+-------+ | 1| 1 | blah | | 2| 1 | blah2 | | 3| 2 | blah3 | +--+------+-------+ I want the results to be: +----+------+ |p.id|p.text| +----+------+ | 1 | Blah | | 2 | Blah2| +----+------+

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• How to Generate XML from Database

  - by Nisarg Mehta

  Hi , I am fetching data from two tables CARRIER_IFTA ,IFTA_NAME. My Select Query is like below.. SELECT t1.IFTA_LICENSE_NUMBER,t1.IFTA_BASE_STATE,t2.NAME_TYPE,t2.NAME from CARRIER_IFTA t1 inner join IFTA_NAME t2 on t1.IFTA_LICENSE_NUMBER=t2.IFTA_LICENSE_NUMBER My Data is coming in this way... IFTA_LICENSE_NUMBER IFTA_BASE_STATE NAME_TYPE NAME -------------------------------------------------------- 630908333 US LG XYZ 630908333 US MG PQR 730908344 US LG ABC Now using XSLT I want to generate XML like this <T0019> <IFTA_ACCOUNT> <IFTA_LICENSE_NUMBER>630908333</IFTA_LICENSE_NUMBER> <IFTA_BASE_STATE>US</IFTA_BASE_STATE> <IFTA_NAME> <NAME_TYPE>LG<NAME_TYPE> <NAME>XYZ</NAME> </IFTA_NAME> <IFTA_NAME> <NAME_TYPE>MG<NAME_TYPE> <NAME>PQR</NAME> <IFTA_NAME> </IFTA_ACCOUNT> <IFTA_ACCOUNT> <IFTA_LICENSE_NUMBER>730908344</IFTA_LICENSE_NUMBER> <IFTA_BASE_STATE>US</IFTA_BASE_STATE> <IFTA_NAME> <NAME_TYPE>LG<NAME_TYPE> <NAME>ABC</NAME> </IFTA_NAME> </IFTA_ACCOUNT> </T0019> i have used below xslt but it is not giveng me desire result ... <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0"> <xsl:template match="/ROWSET"> <xsl:element name="T0019"> <xsl:apply-templates select="IFTAACCOUNT"/> </xsl:element> </xsl:template> <xsl:template match="IFTAACCOUNT"> <xsl:element name="IFTAACCOUNT"> <xsl:apply-templates select="IFTA_CARRIER_ID_NUMBER"/> </xsl:element> </xsl:template> <xsl:template match="IFTA_LICENSE_NUMBER"> <xsl:element name="IFTA_LICENSE_NUMBER"> <xsl:apply-templates /> </xsl:element> </xsl:template> <xsl:template match="IFTA_BASE_STATE"> <xsl:element name="IFTA_BASE_STATE"> <xsl:apply-templates /> </xsl:element> </xsl:template> <xsl:template match="IRP_NAME"> <IRP_NAME> <xsl:apply-templates select="NAME"/> <xsl:apply-templates select="NAME_TYPE"/> </IRP_NAME> </xsl:template> <xsl:template match="NAME"> <xsl:element name="NAME"> <xsl:value-of select="." /> </xsl:element> </xsl:template> <xsl:template match="NAME_TYPE"> <xsl:element name="NAME_TYPE"> <xsl:apply-templates /> </xsl:element> </xsl:template> </xsl:stylesheet> but it is not giving desire result ... Please help me ... Thanks in Advance...

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• How to prune data set by frequency to conform to paper's description

  - by sakura90

  The MovieLens data set provides a table with columns: userid | movieid | tag | timestamp I have trouble reproducing the way they pruned the MovieLens data set used in: Tag Informed Collaborative Filtering, by Zhen, Li and Young In 4.1 Data Set of the above paper, it writes "For the tagging information, we only keep those tags which are added on at least 3 distinct movies. As for the users, we only keep those users who used at least 3 distinct tags in their tagging history. For movies, we only keep those movies that are annotated by at least 3 distinct tags." I tried to query the database: select TMP.userid, count(*) as tagnum from (select distinct T.userid as userid, T.tag as tag from tags T) AS TMP group by TMP.userid having tagnum >= 3; I got a list of 1760 users who labeled 3 distinct tags. However, some of the tags are not added on at least 3 distinct movies. Any help is appreciated.

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• What is wrong with this connection string?

  - by Hakan

  Can any one help me with this connection string. I can't manage how to fix. Dim constring As String Dim con As SqlCeConnection Dim cmd As SqlCeCommand constring = "(System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().GetName().CodeBase) + \\database.sdf;Password=pswrd;File Mode=shared read" con = New SqlCeConnection() con.Open() Thanks

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• How to SELECT DISTINCT Info with TOP 1 Info and an Order By FROM the Top 1 Info

  - by Erin Taylor

  I have 2 tables, that look like: CustomerInfo(CustomterID, CustomerName) CustomerReviews(ReviewID, CustomerID, Review, Score) I want to search reviews for a string and return CustomerInfo.CustomerID and CustomerInfo.CustomerName. However, I only want to show distinct CustomerID and CustomerName along with just one of their CustomerReviews.Reviews and CustomerReviews.Score. I also want to order by the CustomerReviews.Score. I can't figure out how to do this, since a customer can leave multiple reviews, but I only want a list of customers with their highest scored review. Any ideas?

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• Left Join works with table but fails with query

  - by Frank Martin

  The following left join query in MS Access 2007 SELECT Table1.Field_A, Table1.Field_B, qry_Table2_Combined.Field_A, qry_Table2_Combined.Field_B, qry_Table2_Combined.Combined_Field FROM Table1 LEFT JOIN qry_Table2_Combined ON (Table1.Field_A = qry_Table2_Combined.Field_A) AND (Table1.Field_B = qry_Table2_Combined.Field_B); is expected by me to return this result: +--------+---------+---------+---------+----------------+ |Field_A | Field_B | Field_A | Field_B | Combined_Field | +--------+---------+---------+---------+----------------+ |1 | | | | | +--------+---------+---------+---------+----------------+ |1 | | | | | +--------+---------+---------+---------+----------------+ |2 |1 |2 |1 |John, Doe | +--------+---------+---------+---------+----------------+ |2 |2 | | | | +--------+---------+---------+---------+----------------+ [Table1] has 4 records, [qry_Table2_Combined] has 1 record. But it gives me this: +--------+---------+---------+---------+----------------+ |Field_A | Field_B | Field_A | Field_B | Combined_Field | +--------+---------+---------+---------+----------------+ |2 |1 |2 |1 |John, Doe | +--------+---------+---------+---------+----------------+ |2 |2 |2 | |, | +--------+---------+---------+---------+----------------+ Really weird is that the [Combined_Field] has a comma in the second row. I use a comma to concatenate two fields in [qry_Table2_Combined]. If the left join query uses a table created from the query [qry_Table2_Combined] it works as expected. Why does this left join query not give the same result for a query and a table? And how can i get the right results using a query in the left join?

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• Custom SQL function for NHibernate dialect

  - by Kristoffer Ahl

  I want to be able to call a custom function called "recent_date" as part of my HQL. Like this: [Date] >= recent_date() I created a new dialect, inheriting from MsSql2000Dialect and specified the dialect for my configuration. public class NordicMsSql2000Dialect : MsSql2000Dialect { public NordicMsSql2000Dialect() { RegisterFunction( "recent_date", new SQLFunctionTemplate( NHibernateUtil.Date, "dateadd(day, -15, getdate())" ) ); } } var configuration = Fluently.Configure() .Database( MsSqlConfiguration.MsSql2000 .ConnectionString(c => .... ) .Cache(c => c.UseQueryCache().ProviderClass<HashtableCacheProvider>()) .Dialect<NordicMsSql2000Dialect>() ) .Mappings(m => ....) .BuildConfiguration(); When calling recent_date() I get the following error: System.Data.SqlClient.SqlException: 'recent_date' is not a recognized function name I'm using it in a where statement for a HasMany-mapping like below. HasMany(x => x.RecentValues) .Access.CamelCaseField(Prefix.Underscore) .Cascade.SaveUpdate() .Where("Date >= recent_date()"); What am I missing here?

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• Is there a way to select nth column in a SELECT clause from a table/view

  - by Vishal Seth

  I've this GIGANTIC view with several hundred columns and I need to select 114th column something like: SELECT "144" FROM MyView; PS: Obviously, I don't know the name of the column. I just copied the results row into an Excel file, searched for a particular value that is in EJ column so I want to select all rows showing only nth column in my view to do my further debugging. Cheers!

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• Native XML WebService With Authentication Basic and SSL

  - by tom

  I'm using 2005 and the Native XML WebServices. The integrated authentication via HTTP:80 works fine. But i need the basic authentication which requires SSL. So if i change the web service to ssl i always get a connection reset (101). I tried several ports 80,443,9999 with the same outcome. What is the error?

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• How to run an .exe application in another computer?

  - by ADAM

  I am working on a C# application in Visual Studio 2013. When I run the .exe file from my computer, the application runs very well and all the features work. When I tried to run the .exe on another computer, the database side doesn't work well and the connection with the database couldn't be opened. The SqlConnection is constructed as follows: SqlConnection cn = new SqlConnection("Data Source=ADAM-PC;Initial Catalog=integrationdatabase;Integrated Security=True" I don't know how to change the data source to make the connection with the database established in another computer. How can I solve this problem?

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• Is there an automatic modification time stamp type for Oracle columns?

  - by Ayrad

  Is there a way to create a timestamp column in Oracle that automatically stores a timestamp of when the record has changed ?

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• select from varchar2 column with numeric value sometimes gives invalid number error

  - by Rene

  I'm trying to understand why, on some systems, I get an invalid number error message when I'm trying to select a value from a varchar2 column while on other systems I don't get the error while doing the exact same thing. The table is something like this: ID Column_1 Column_2 1 V text 2 D 1 3 D 2 4 D 3 and a query: select ID from table where column_1='D' and column_2 = :some_number_value :some_number_value is always numeric but can be null. We've fixed the query: select ID from table where column_1='D' and column_2 = to_char(:some_number_value) This original query runs fine on most systems but on some systems gives an "invalid number" error. The question is why? Why does it work on most systems and not on some?

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• SqlCE DB occasionally freezes on one handheld, not another

  - by Michael

  I have two types of custom handhelds which are similar, but slightly different, each running the same WinForm application and a WinCE database: Type 1: WinCE 4.2, 400 mhz, 93244 kb Type 2: WinCE 5.0, 520 mhz, 84208 kb Type 1 will happily proceed through a large batch db operation (initiated) by the app, by Type 2 will consistently begin c-r-a-w-l-i-n-g (for several to many cycles) at around the 200 cycle mark. As several points it will begin running normally and then crawl again. The app does several db op's (inserts, updates and selects, no deletes). To simplify my situation, I've built a small test app which essentially does this: command_s.CommandText = "select dvr from vr where vid = 2211250"; command_u.CommandText = "update pvr set LocationID=81 where Status='OK' and vri = 27861"; while(going) { command_s.ExecuteScalar(); command_u.ExecuteNonQuery(); } and set it off running on the two units side by side. Sure enough, the slower (400 mhz) unit is outpacing the faster (520 mhz) unit (it's about 5000 cycles ahead right now) and I can see noticable pauses on the 520 mhz unit. What is causing this?

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• validate linqtosql mapping to a model

  - by Coppermill

  I have generated a LinqtoSQL mapping xml file, which I have a valid XSD schema that I check to make sure the XML is correct. Now I want to check that the field type match the Model/Interface for example: checking that the nullable fields are nullable that int are int etc anyone got any ideas if I can do this?

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• How to combine a list of choices to determine which select statement

  - by Larry

  I have a mysql db and am using php 5.2 What I am trying to do is offer a list of options for a person to select (only 1). The chosen option will cause a select, update, or delete statement to be ran. The results of the statement do not need to be shown, although, showing the old and then the new would be nice (no problems with that part tho'.). Pseudo-Code: Assign $choice = 0 Check the value of $choice // This way, if it = 100, we do a break Select a Choice:<br> 1. Adjust Status Value (+60) // $choice = 1<br> 2. Show all Ships <br> // $choice = 2 3. Show Ships in Port <br> // $choice = 3 ... 0. $choice="100" // if the value =100, quit this part Use either case (switch) or if/else statements to run the users choice1 If the choice is 1, then run the "Select" statement with the variable of $sql1 -- "SELECT .... If the choice is 2, then run the "Select" statement with the variable of $sql2 --- SELECT * FROM Ships If the choice is 3, then run the "Select" statement with the variable of $sql3 <br> .... If the choice is 0, then we are done. I figured the (3) statements would be assigned in php as: $sql1="...". $sql2="SELECT * FROM Ships" $sql3="SELECT * FROM Ships WHERE nPort="1" My idea was to use the switch statement, but got lost on it. :( I would like the options to be available over and over again, until a variable ($choice) is selected. In which case, this particular page is done and goes back to the "Main Menu"? The coding and display, if I use it, I can do. Just not sure how to write the way to select which one I want. It is possible that I would run all of the queries, and other times, only one, so that is why I would like the choice. An area I get confused in is the proper forms to use such as -- ' ' " " and ...?? Not sure the # of options I will end up with, but it will be more than 5 but less than 20 / page. So if I get the system down for 2-3 choices, I can replicate it for as many as I may need. And, as always, if a better way exists, I am willing to try it. Thanks again... Larry

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• PostgreSQL - select only when specific multiple apperance in column

  - by Horse SMith

  I'm using PostgreSQL. I have a table with 3 fields person, recipe and ingredient person = creator of the recipe recipe = the recipe ingredient = one of the ingredients in the recipe I want to create a query which results in every person who whenever has added carrot to a recipe, the person must also have added salt to the same recipe. More than one person can have created the recipe, in which case the person who added the ingredient will be credited for adding the ingredient. Sometimes the ingredient is used more than once, even by the same person. If this the table: person1, rec1, carrot person1, rec1, salt person1, rec1, salt person1, rec2, salt person1, rec2, pepper person2, rec1, carrot person2, rec1, salt person2, rec2, carrot person2, rec2, pepper person3, rec1, sugar person3, rec1, carrot Then I want this result: person1 Because this person is the only one who whenever has added carrot also have added salt.

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• LINQ Group By to project into a non-anonymous type?

  - by vikp

  Hi, I have the following LINQ example: var colorDistribution = from product in ctx.Products group product by product.Color into productColors select new { Color = productColors.Key, Count = productColors.Count() }; All this works and makes perfect sense. What I'm trying to achieve is to group by into a strong type instead of anonymous type. For example I have a ProductColour class and I would like to Group into a List<ProductColour> Is this possible? Thank you

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• How can I compare two sets of data in linq and return the common data?

  - by kjm

  I have two IList<string> a and b. I want to find out what strings are in both a and b using LINQ.

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• Can I chain database links in Oracle?

  - by RenderIn

  I have 3 databases. 1 links to 2, 2 links to 3. I'd like to query tables in 3, from 1. I tried third_db_tab@3@2 and it did not work. Wondering if this is possible and if so, what the syntax is.

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• What's the difference between tsql and isql?

  - by wamp

  I saw this in this post,and is really confused

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• Postgresql count+sort performance

  - by invictus

  I have built a small inventory system using postgresql and psycopg2. Everything works great, except, when I want to create aggregated summaries/reports of the content, I get really bad performance due to count()'ing and sorting. The DB schema is as follows: CREATE TABLE hosts ( id SERIAL PRIMARY KEY, name VARCHAR(255) ); CREATE TABLE items ( id SERIAL PRIMARY KEY, description TEXT ); CREATE TABLE host_item ( id SERIAL PRIMARY KEY, host INTEGER REFERENCES hosts(id) ON DELETE CASCADE ON UPDATE CASCADE, item INTEGER REFERENCES items(id) ON DELETE CASCADE ON UPDATE CASCADE ); There are some other fields as well, but those are not relevant. I want to extract 2 different reports: - List of all hosts with the number of items per, ordered from highest to lowest count - List of all items with the number of hosts per, ordered from highest to lowest count I have used 2 queries for the purpose: Items with host count: SELECT i.id, i.description, COUNT(hi.id) AS count FROM items AS i LEFT JOIN host_item AS hi ON (i.id=hi.item) GROUP BY i.id ORDER BY count DESC LIMIT 10; Hosts with item count: SELECT h.id, h.name, COUNT(hi.id) AS count FROM hosts AS h LEFT JOIN host_item AS hi ON (h.id=hi.host) GROUP BY h.id ORDER BY count DESC LIMIT 10; Problem is: the queries runs for 5-6 seconds before returning any data. As this is a web based application, 6 seconds are just not acceptable. The database is heavily populated with approximately 50k hosts, 1000 items and 400 000 host/items relations, and will likely increase significantly when (or perhaps if) the application will be used. After playing around, I found that by removing the "ORDER BY count DESC" part, both queries would execute instantly without any delay whatsoever (less than 20ms to finish the queries). Is there any way I can optimize these queries so that I can get the result sorted without the delay? I was trying different indexes, but seeing as the count is computed it is possible to utilize an index for this. I have read that count()'ing in postgresql is slow, but its the sorting that are causing me problems... My current workaround is to run the queries above as an hourly job, putting the result into a new table with an index on the count column for quick lookup. I use Postgresql 9.2.

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