DbSchema Tutorial with Introduction in SQL Databases

Transcription

1 DbSchema Tutorial with Introduction in SQL Databases Contents Connect to the Database and Create First Tables... 2 Create Foreign Keys... 7 Create Indexes... 9 Generate Random Data Relational Data Browse DbSchema Project File Schema Synchronization DbSchema Layouts Visual Query Builder The SQL Editor Forms and Reports Technical Support End In this tutorial you will get an introduction into relational databases. Among the tutorial we will use the DbSchema designer tool. Download, install and open DbSchema from DbSchema can be used on trial base 15 days for free.

2 Connect to the Database and Create First Tables Start DbSchema and you should get this screen. Choose Connect to Database. Choose this We are going to connect to a H2 database. We will use this because H2 requires no other software to be installed. For MySql, MariaDb, Postgresql, etc., the database software should be installed as well. If you wish to do this please check in DbSchema Help, in the Databases and How to Connect there are some tips, they will help to install the database software end enable network connections so you can connect DbSchema to the database. H2 connection to file database Press new to create a new database in a folder

3 In the connection dialog choose first H2 database. This will prompt to download the driver. Choose Download from DbSchema. Press Create New to create an empty database. In the next dialog you can choose or create an empty folder. Create new folder Press Ok in the connection dialog. Next step is to choose a schema you will work on. A schema is similar with a group of the tables, so different applications may use the same database but different schemes. Information_schema is an internal H2 schema, with tables used by H2 itself. Press ok and we should get a screen like bellow. The database is empty, no table is created. Create a table by right-clicking the layout (the large free space) and Create table.

4 Right-click the layout and choose Create table. In our tutorial we are going to create two tables: Customers and Employees. Each table will have few columns. Customers Customerid a number, to identify each customer. For example a customer Targot will have the customerid 100, and each time we reffer the customerid 100 we know we talk about this customer. Using this Id s has the advantage that we can easy rename the customer and all other tables which point to it won t be changed. Name some text Description - text Employees EmployeeId similar with customerid, a number to identify each employee Firstname text Lastname test Birthdate date Companyid the id of the company he or she works for In the table dialog enter the first table name, than add one column at a time by pressing the add button.

5 Enter the table name Add columns one by one When you will enter the table name as well as the column names, you will notice the text is automatically converted to upper-cases. This because the database is using by default uppercases for table and column names, and DbSchema convert them to show them as they are looking in the database. Let s add the first column CUSTOMERID. Enter the name, choose the data type Integer which means number. The checkbox Primary Key should be on, which means this column will unique identify any record from this table. Primary keys must always be specified. This means all records should have a value in this field and therefore the mandatory field is checked and cannot be un-checked.

6 Similar we add the name and description as varchar (this is text) with maximal number of characters allowed 100 for name and 500 for description. The name should be mandatory in order to make sure no company is missing the name. After adding all columns the table will look like here: Primary key and x for mandatory symbols Or like this: Press this button to see the data type

7 Similar create the second table EMPLOYEES. Create Foreign Keys To prevent having employees with CUSTOMERID values which does not exists in the customer table, we are going to create a foreign key. The foreign key is a constraint, which means it is a logical verification of the data. This verification takes place each time a new record is inserted in the table or the data is modified. To create the foreign key, drag and drop the CUSTOMERID from EMPLOYEES over the CUSTOMERID in CUSTOMERS. We want that each EMPLOYEE.CUSTOMERID exists in the CUSTOMERS.CUSTOMERID. Drag & drop column The foreign key dialog will open:

8 The foreign key requires the referring table (employees) and referred table (customers) plus the columns for each of them. We can set an action to be taken if a customer is deleted. No action means nothing will be done. If we will try to drop a customer which has employees we will get back an error message. We can set on delete cascade and all employees belonging to a customer will be deleted when the company is deleted. Virtual means the foreign key is created only in DbSchema. In this case no data verification will be done in the database. The virtual foreign keys may be used only for design purposes or for relational data browse which will be explained later. A foreign key always require an index to exist on the referred columns (CUSTOMERID in customers table). If this column is a primary key will automatically get an index. Read more about indexes in the index chapter. About the foreign key impact on performance Is a common mistake to skip creating foreign keys because they slow down the database. This is not true. The foreign keys are a validation of the data which is done only when inserting or deleting new

9 rows. Usually the ID fields don t get modified, so the foreign key won t be triggered. The foreign key validation won t be done when updating other columns than the foreign key columns. Foreign keys are useful as they will prevent having bad data in the database. This is very important! Foreign keys are healthy! Create Indexes Database indexes are used to fasten searching of data by a given criteria. Consider a query SELECT * FROM EMPLOYEES WHERE FIRSNAME= JOHN. Without indexes the database will scan the complete data before finding the employees with this firstname. This may take time if the table contains many records. Having an index on FIRSTNAME will fasten the search. Indexes work similar with a book keyword index, which can be usually found in the end of a book. An index is a list of values in a column with a reference to the position where the values can be found in the table. Indexes duplicate the column data in a separate structure and hold for each distinct value a list of pointers to the table records. The index is synchronized by the database server each time the column data is modified, inserted or deleted. Let s create an index on EMPLOYEES.FIRSTNAME. Right-click the FIRSTNAME column and choose Add Index.

10 An index can be normal, which means any value can be stored inside. Unique indexes do not allow duplicate values. When a value is inserted in the table field and that value exists in another record for the same field, the insert operation will throw an error and the operation is aborted. Primary Key indexes are the same as unique indexes plus the condition that the column must be mandatory (does not allow missing or empty values). A table can have at most one primary key, but if you need more you can create unique indexes and set the columns to be mandatory. This has the same effect as creating a primary key column. After creating the index a symbol will show in front of the column. Index symbol Indexes require more space in the database and decrease the database performance on inserts and updates, but increase the performance by selects. In case of foreign keys with on delete cascade is recommended to set-up an index on the referring column ( customerid in employees ). This will fasten the deletion of any customer from customer table. Queries can use at most one index for a table. If for example you want to search an employee by firstname and lastname and you have one index for firstname and another index for lastname, only one index will be used. One solution is to create a compound index on two or more columns. Create an index on firstname and lastname in the same time. This will help on searching by firstname and lastname, or only by firstname. The index won t help searching only on lastname - either the first or

11 first and second columns have to be specified in the search criteria. Having a value only for the second column won t use the index. Generate Random Data Let s fill the tables with some random data. For this choose the random data generator from the menu. Generate ramdom data Generate data In the dialog press Generate to start the generator. Optional we can configure other patterns for the fields as the default settings. Double-click one of the tables to open the patterns editor for each of the columns. Relational Data Browse DbSchema has a dedicated tool for exploring and editing the data. Is called relational because can explore data from multiple tables based on the foreign keys between them.

12 Right-click on CUSTOMERS table header and press the Relational Data Browse from the popup. The option is available in the right-click pop-up as well. Relational Data Browse The editor will open. We can descend in the EMPLOYEES table by pressing the foreign key arrow in the table header. Choose Join EMPLOYEES. Descend into EMPLOYEES Now we are browsing two tables. Select a different record in the first editor will refresh the second with the matching rows based on foreign key, which are the matching employees for the selected company.

13 To filter a column data click the column header. To edit the data double-click any cell. Double-click cell to edit. Add or delete row

14 DbSchema Project File Our work including the schema, diagrams, SQL Editors and relational data browse can be saved to project file. The file has the extension.dbs. Save project to file. Next time when you restart DbSchema, the project file will be reopened. If you are working with a dedicated database server and you are in a location where you cannot connect to the database, the project file can still be opened and the diagrams will show. The relational data browse cannot be used because the data has to be loaded from the database.

15 Schema Synchronization DbSchema is using its own image of the schema. This is what is being saved in the project file. Whenever you connect to the database you have to press the refresh button to make sure the internal schema is the same as in the database. Refresh schema Online or offline DbSchema can be used online (connected to database) or offline. Switching to online will require refreshing schema from the database or creating schema in the database (if you designed a fresh schema directly in DbSchema offline). After refresh you may decide which differences to take over in DbSchema internal project or apply in the database. If you design a schema offline you can connect to any database and create the schema in the database. You may improve an existing schema, connect and deploy the latest changes on different databases.

16 DbSchema Layouts A layout is the diagram and the associated tools opened inside ( Relational data browse, SQL editor, Query builder, Database reports, etc. ). Create further layouts from the menu. Create further layouts One table may show in different layouts or in none of them. Adding tables to layout can be done by drag & drop or by pressing the foreign key icon beside columns. Drag & drop tables to layout from here Add further tables to layout by pressing the foreign key icon

17 Visual Query Builder Relational data browse is one tool for accessing the data. Beside it we can use the SQL Editor and the Query builder. Start the query builder by clicking the table header. The builder will open with the selected table. We are going to build a query over both tables. We add the CUSTOMERS table by clicking the foreign key icon. Foreign key icon Tick the checkboxes for the columns we want to show in the result. To set a filter (where clause) right click a column. Execute the query. The result can be saved to file. The generated query is on the left, in SQL Preview frame.

18 Execute the query Generated query The SQL Editor Use the SQL editor will to edit and execute SQL queries. Text auto-completion is enabled on CTRL- BLANK keys.

19 Forms and Reports Using DbSchema you can create small applications or database reports. Start a report from the application menu, edit the query in the query builder (called data source, add tables to it by drag & drop). After editing and executing the query the wizard will continue. One more screen to select the components to add to report: and the report is ready to be executed from the menu.

20 Execution mode: HTML or swing Run report Execute the report and the web browser will show the data. On DbSchema website there are dedicated tutorials for learning forms and reports. Read them and you will discover how easy is to create a master-detail report, do customization, etc. Technical Support On DbSchema website there are contact forms where you can write us with any technical questions. Unregistered users may ask as well. Don t hesitate to write! End Thank you for taking this course. Please give us feedback about DbSchema, your help is appreciated!