Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. Qt Development
  3. Mobile and Embedded
  4. Advice about the application I will be making? (I'm a beginner)
Forum Updated to NodeBB v4.3 + New Features

Advice about the application I will be making? (I'm a beginner)

Scheduled Pinned Locked Moved Unsolved Mobile and Embedded
77 Posts 6 Posters 25.4k Views 2 Watching
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • mrjjM Offline
    mrjjM Offline
    mrjj
    Lifetime Qt Champion
    wrote on last edited by
    #52

    Hi
    I cant see anything wrong.
    Show how code is before the if ( mydb.open )

    D 1 Reply Last reply
    1
    • mrjjM mrjj

      Hi
      I cant see anything wrong.
      Show how code is before the if ( mydb.open )

      D Offline
      D Offline
      dvlpr.bernard
      wrote on last edited by
      #53

      The code above is inside of this button function
      0_1556624873643_3c1572d4-d1c4-4bcb-be5b-7c7a6701dfb4-image.png

      Here's the declaration of the database
      0_1556624942089_c2a56415-3c65-44f0-bc2b-e108db11ebed-image.png
      0_1556624974330_fa46ea4d-303f-475f-b64e-b252d5391cfb-image.png

      JonBJ 1 Reply Last reply
      0
      • D dvlpr.bernard

        The code above is inside of this button function
        0_1556624873643_3c1572d4-d1c4-4bcb-be5b-7c7a6701dfb4-image.png

        Here's the declaration of the database
        0_1556624942089_c2a56415-3c65-44f0-bc2b-e108db11ebed-image.png
        0_1556624974330_fa46ea4d-303f-475f-b64e-b252d5391cfb-image.png

        JonBJ Offline
        JonBJ Offline
        JonB
        wrote on last edited by
        #54

        @dvlpr.bernard

        • I notice that your Output window shows two occurrences of your message Connected.... Why is that? Are you connecting more than once?? [In a separate issue, in the long run you absolutely will not want to open() and then close() the database for each query. The whole point is to connect once, keep the connection open while you execute all your queries, and close when you are completely done with the database.]

        • [...] but it didn't work also.

        What does that mean? Error message? Behaviour? What?? Please help us to help you by supplying useful information.

        D 1 Reply Last reply
        1
        • JonBJ JonB

          @dvlpr.bernard

          • I notice that your Output window shows two occurrences of your message Connected.... Why is that? Are you connecting more than once?? [In a separate issue, in the long run you absolutely will not want to open() and then close() the database for each query. The whole point is to connect once, keep the connection open while you execute all your queries, and close when you are completely done with the database.]

          • [...] but it didn't work also.

          What does that mean? Error message? Behaviour? What?? Please help us to help you by supplying useful information.

          D Offline
          D Offline
          dvlpr.bernard
          wrote on last edited by
          #55

          @JonB @mrjj
          if (query.exec( ))
          always returns a false value thats why the query never gets executed.
          0_1556682147823_32f7bcf9-5b1b-405b-b851-94dd8fd9cf15-image.png
          0_1556682115403_04ffb565-381f-4719-8260-9fac15c4cc3b-image.png

          1 Reply Last reply
          0
          • D Offline
            D Offline
            dvlpr.bernard
            wrote on last edited by
            #56

            I played around with the code and this error return.

            0_1556684436755_30f80dab-2418-4b5f-8d3a-5ac9047df238-image.png

            1 Reply Last reply
            0
            • D Offline
              D Offline
              dvlpr.bernard
              wrote on last edited by
              #57

              0_1556685583290_a11aff6b-ff9a-4e92-828b-3de115afe4c9-image.png

              JonBJ KillerSmathK 2 Replies Last reply
              0
              • mrjjM Offline
                mrjjM Offline
                mrjj
                Lifetime Qt Champion
                wrote on last edited by mrjj
                #58

                hi
                are you sure the table name is mydatabase ?
                seems wrong. unless the table is called taht.
                But it seems you used the name of the database.

                and it even says unknown table.
                so use table name instead.

                1 Reply Last reply
                3
                • D dvlpr.bernard

                  0_1556685583290_a11aff6b-ff9a-4e92-828b-3de115afe4c9-image.png

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by JonB
                  #59

                  @dvlpr.bernard

                  no such table: mydatabase

                  @mrjj wrote:

                  are you sure the table name is mydatabase ?

                  Are you getting your database vs your tables mixed up in your head? You work with one database at a time. In SQLite that corresponds to the file you pass to setDatabaseName(), and is what gets opened via mydb.open(). A database may contain many tables, each with their own name. It is the table name you mention in SQL select (and most other) statements. You previously created your table(s) via some kind of CREATE TABLE name ... statement, and that name is what you need in your query. It would be "unusual" for you to have named a table mydatabase.

                  You might like to do yourself a favour, to help with all the SQLite queries you're trying to write. Download one of the (free) "workbenches" for SQLite. These are GUI applications which will let you see what is in your database; it will also let you construct and play with queries, till you are ready to copy them over to your Qt code. Google for sqlite workbench, there seem to be several, e.g. https://www.sqlite-workbench.com/, https://sqlitestudio.pl, https://alternativeto.net/software/sqlite-manager/. I suggest you take the time to pick one, in the long run I think the effort will be well worthwhile for you.

                  1 Reply Last reply
                  2
                  • D Offline
                    D Offline
                    dvlpr.bernard
                    wrote on last edited by
                    #60

                    Good day.
                    So here is my database named codeanaldb with a table name "mydatabase". I didn't know why qt couldn't recognize it.

                    0_1556721420905_3aa67a8d-a82e-4456-9548-1500489ec844-image.png

                    And here is the same query that I copy paste to qt but suddenly didn't work.

                    0_1556721664803_4baa7258-307d-4a59-9dbb-68db56cb80f8-image.png

                    JonBJ 1 Reply Last reply
                    0
                    • D dvlpr.bernard

                      Good day.
                      So here is my database named codeanaldb with a table name "mydatabase". I didn't know why qt couldn't recognize it.

                      0_1556721420905_3aa67a8d-a82e-4456-9548-1500489ec844-image.png

                      And here is the same query that I copy paste to qt but suddenly didn't work.

                      0_1556721664803_4baa7258-307d-4a59-9dbb-68db56cb80f8-image.png

                      JonBJ Offline
                      JonBJ Offline
                      JonB
                      wrote on last edited by
                      #61

                      @dvlpr.bernard
                      In a word, Qt will "accept" any SQL query you can make work in a workbench. If it isn't something else is wrong, e.g. you're not connected to the database you think you are....

                      1 Reply Last reply
                      1
                      • D dvlpr.bernard

                        0_1556685583290_a11aff6b-ff9a-4e92-828b-3de115afe4c9-image.png

                        KillerSmathK Offline
                        KillerSmathK Offline
                        KillerSmath
                        wrote on last edited by
                        #62

                        @dvlpr.bernard said in Advice about the application I will be making? (I'm a beginner):

                        0_1556685583290_a11aff6b-ff9a-4e92-828b-3de115afe4c9-image.png

                        Are you sure that your database file is Database codeanaldb ? Because in last image you have used Database codeanal.db (with .)

                        @Computer Science Student - Brazil
                        Web Developer and Researcher
                        “Sometimes it’s the people no one imagines anything of who do the things that no one can imagine.” - Alan Turing

                        1 Reply Last reply
                        2
                        • D Offline
                          D Offline
                          dvlpr.bernard
                          wrote on last edited by dvlpr.bernard
                          #63

                          Holly cow, after more than a 100 trial and error, I got it guys. So the problem is I didn't set the path correctly but I'm pretty sure I set it correctly earlier but then other code wasn't correct so I played around and tried change the path, I don't know why it is still working even if the path is wrong. So thank you again guys :).

                          Correct One: - connected and query is worling
                          mydb.setDatabaseName("C:\Users\bj\Documents\CODEANALL\Database\codeanaldb1.db");
                          Wrong One: - connected but the query isn't working dunno why
                          mydb.setDatabaseName("C:\Users\bj\Documents\CODEANALL\Database codeanaldb1.db");

                          0_1556722834541_bdf923ef-e973-41f0-ae89-f8e84f224c65-image.png

                          JonBJ KillerSmathK 2 Replies Last reply
                          0
                          • D dvlpr.bernard

                            Holly cow, after more than a 100 trial and error, I got it guys. So the problem is I didn't set the path correctly but I'm pretty sure I set it correctly earlier but then other code wasn't correct so I played around and tried change the path, I don't know why it is still working even if the path is wrong. So thank you again guys :).

                            Correct One: - connected and query is worling
                            mydb.setDatabaseName("C:\Users\bj\Documents\CODEANALL\Database\codeanaldb1.db");
                            Wrong One: - connected but the query isn't working dunno why
                            mydb.setDatabaseName("C:\Users\bj\Documents\CODEANALL\Database codeanaldb1.db");

                            0_1556722834541_bdf923ef-e973-41f0-ae89-f8e84f224c65-image.png

                            JonBJ Offline
                            JonBJ Offline
                            JonB
                            wrote on last edited by JonB
                            #64

                            @dvlpr.bernard

                            Correct One: - connected and query is worling
                            mydb.setDatabaseName("C:\Users\bj\Documents\CODEANALL\Database\codeanaldb1.db");
                            Wrong One: - connected but the query isn't working dunno why
                            mydb.setDatabaseName("C:\Users\bj\Documents\CODEANALL\Database codeanaldb1.db");

                            I suspect (never having used SQLite) that your existing database is the first path. When you specify the second path to open(), being non-existent SQLite decides to create that for you, as an empty database, which then does not have the table mentioned in the query/error message. Check your disk (directly in the CODEANALL folder) to see if the second one exists, or exists so long as you are running the query?

                            If that is the case, there "must" be some way of asking to open only an existent database/file from SQLite/Qt which you could use for safety?

                            1 Reply Last reply
                            1
                            • D dvlpr.bernard

                              Holly cow, after more than a 100 trial and error, I got it guys. So the problem is I didn't set the path correctly but I'm pretty sure I set it correctly earlier but then other code wasn't correct so I played around and tried change the path, I don't know why it is still working even if the path is wrong. So thank you again guys :).

                              Correct One: - connected and query is worling
                              mydb.setDatabaseName("C:\Users\bj\Documents\CODEANALL\Database\codeanaldb1.db");
                              Wrong One: - connected but the query isn't working dunno why
                              mydb.setDatabaseName("C:\Users\bj\Documents\CODEANALL\Database codeanaldb1.db");

                              0_1556722834541_bdf923ef-e973-41f0-ae89-f8e84f224c65-image.png

                              KillerSmathK Offline
                              KillerSmathK Offline
                              KillerSmath
                              wrote on last edited by KillerSmath
                              #65

                              @dvlpr.bernard

                              I don't know why it is still working even if the path is wrong.

                              it is because, the QSqliteDriver uses by default, SQLITE_OPEN_CREATE flag to create the file if it is not exists.

                              @Computer Science Student - Brazil
                              Web Developer and Researcher
                              “Sometimes it’s the people no one imagines anything of who do the things that no one can imagine.” - Alan Turing

                              JonBJ 1 Reply Last reply
                              3
                              • KillerSmathK KillerSmath

                                @dvlpr.bernard

                                I don't know why it is still working even if the path is wrong.

                                it is because, the QSqliteDriver uses by default, SQLITE_OPEN_CREATE flag to create the file if it is not exists.

                                JonBJ Offline
                                JonBJ Offline
                                JonB
                                wrote on last edited by JonB
                                #66

                                @KillerSmath
                                Then the question is: can you specify something at open() time to override that behaviour, without changing the driver? Or, can you temporarily change that driver flag from code before opening? Then the OP would have found out/use it in future.

                                Meanwhile, the OP needs to understand that the two paths (one with space, one with backslash) refer to quite different files!

                                D KillerSmathK 2 Replies Last reply
                                1
                                • JonBJ JonB

                                  @KillerSmath
                                  Then the question is: can you specify something at open() time to override that behaviour, without changing the driver? Or, can you temporarily change that driver flag from code before opening? Then the OP would have found out/use it in future.

                                  Meanwhile, the OP needs to understand that the two paths (one with space, one with backslash) refer to quite different files!

                                  D Offline
                                  D Offline
                                  dvlpr.bernard
                                  wrote on last edited by dvlpr.bernard
                                  #67

                                  I see so that's the reason why there is a newly created database in my folder. Also backslash "\" specifies the parent folder. I have encountered that on web dev. Thank you!

                                  1 Reply Last reply
                                  0
                                  • JonBJ JonB

                                    @KillerSmath
                                    Then the question is: can you specify something at open() time to override that behaviour, without changing the driver? Or, can you temporarily change that driver flag from code before opening? Then the OP would have found out/use it in future.

                                    Meanwhile, the OP needs to understand that the two paths (one with space, one with backslash) refer to quite different files!

                                    KillerSmathK Offline
                                    KillerSmathK Offline
                                    KillerSmath
                                    wrote on last edited by
                                    #68

                                    @JonB
                                    Your answer is on sqlite source code.

                                    Unhapply, that behaviour is only avoided if you are using SQLITE_OPEN_READONLY flag

                                    @Computer Science Student - Brazil
                                    Web Developer and Researcher
                                    “Sometimes it’s the people no one imagines anything of who do the things that no one can imagine.” - Alan Turing

                                    JonBJ 1 Reply Last reply
                                    3
                                    • KillerSmathK KillerSmath

                                      @JonB
                                      Your answer is on sqlite source code.

                                      Unhapply, that behaviour is only avoided if you are using SQLITE_OPEN_READONLY flag

                                      JonBJ Offline
                                      JonBJ Offline
                                      JonB
                                      wrote on last edited by JonB
                                      #69

                                      @KillerSmath
                                      Then @dvlpr-bernard might want to do a QFile::exists() check on the filepath prior to the open() if he wants to ensure he is opening a correctly-spelled existing database rather than accidentally creating a blank new one....

                                      1 Reply Last reply
                                      3
                                      • D Offline
                                        D Offline
                                        dvlpr.bernard
                                        wrote on last edited by
                                        #70

                                        I tried different code but it didn't work. What should I do to replace query variable value with my variable in qt.
                                        "select Answer from mydatabase where QQuantity = 5 and Topic = 'FOR' and Difficulty_Level = 'ADVANCED'"
                                        0_1556809488139_40bef343-e9e0-41ad-bb2e-070a5789455e-image.png

                                        KroMignonK 1 Reply Last reply
                                        0
                                        • D dvlpr.bernard

                                          I tried different code but it didn't work. What should I do to replace query variable value with my variable in qt.
                                          "select Answer from mydatabase where QQuantity = 5 and Topic = 'FOR' and Difficulty_Level = 'ADVANCED'"
                                          0_1556809488139_40bef343-e9e0-41ad-bb2e-070a5789455e-image.png

                                          KroMignonK Offline
                                          KroMignonK Offline
                                          KroMignon
                                          wrote on last edited by KroMignon
                                          #71

                                          @dvlpr.bernard This should work:

                                          query.prepare("select answer from mydatabase where QQuantitiy=:qquantity AND Topic =:topic and Difficulty_Level =:difficulty_level")
                                          query.bindValue(":qquantity", current_num);
                                          query.bindValue(":topic", ch_topic);
                                          query.bindValue(":difficulty_level", ch_div_lvl);
                                          

                                          binValue will add necessary escape sequencies for string values.
                                          you only have to ensure placeholder in prepare string is the same as which given to bindValue().

                                          Read QSqlQuery Class for more details.

                                          It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                                          D 1 Reply Last reply
                                          4

                                          • Login

                                          • Login or register to search.
                                          • First post
                                            Last post
                                          0
                                          • Categories
                                          • Recent
                                          • Tags
                                          • Popular
                                          • Users
                                          • Groups
                                          • Search
                                          • Get Qt Extensions
                                          • Unsolved